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OP 1«E

iRsiTY OF California.

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MEC HANICA L ENGINEERING

NET BOOK. — This book is supplied to the
Trade on terms which will not allow of Discount
to the Public.

CHARLES GRIFFIN & CO., LTD.

MECHANICAL ENGINEERING
FOR BEGINNERS.

GRIFFIN'S STUDENTS' TEXT-BOOKS.

In Crown 8to. Pp. i-xiii + 466, with 8 Folding Plates and 209 Illustrations in the
Text. Ob. net.

PRACTICAL CALCULATIONS FOR ENGINEERS.

Br CHARLES E. LARARD, A.M.In8T.C.E., M.I.Mbch.E., &c.,
Head of the Mechanical Engineering Department at the Northampton Institute,

London, E.G.
" Admirable . . . exactly what it should be in order to make it useful to students
and practitioners of engineering."— Manchester Ouardian.

In Large Grown 8vo. Cluth. Pp. i-viii+ 143, with 67 Illustrations. U. 6d. net.

THE THERMO-DTNAMIC PRINCIPLES OF ENGINE DESIGN.

By LIONEL M. HOBBS, Engb..Lieut. R.N.

"The whole subject has been verv methodically dealt with throughout. ... A
standard reference."— Practical Engineer.

FOUBTH Edition. In Two Parts, Published Separately.
A TEXT-BOOK OF

ENGINEERING DRAVTING AND DESIGN.

By SIDNEY H. WELLS, Wh.Sc, A.M.LC.E., A.M.LMech.E.
Vol. I.— Practical Geometry, Plane, and Solid. Fifth Edition.

4s. 6d.
Vol. II. — Machine and Engine Drawing and Design. Fourth
Edition. 4s. 6d.
"A capital text-book, arranged on an excellent system."— Electrical Review.

In Large Crown 8vo. Handsome Cloth. With 201 Illustrations. 68. net.
AN INTRODUCTION TO

GIRDERS AND COLUMNS,
1 19. Maolmlxaeai s^xad. S«viao«iaa*«ai.

With Examples in Graphic Statics.
By WILLIAM H. ATHERTON, M.Sc, M.LMech.E.
*'A very useful source of information. ... A work which we commend very
highly."— Nature.

\yoPks by A NDRBW^ JAMIBSON, M .Inst.C,B^ &e>

Eleybnth Edition. Revised and Enlarged. With B. of E., C. and G., and other

Exam. Questions. Ss. 6d.

AN ELEMENTARY MANUAL OP

STEAM AND THE STEAM ENGINE.

"Should be in the hands of every engmeering apprentice."— Procticai Engineer.

EiQHTH Edition, Revised and greatly Enlarged. With many Exam. Questions.

3s. 6d.
AN ELEMENTARY MANUAL OP

ji^F:E>XjX':HiJD Ds^EOH-A^nsrios.

' A thoroughly trustworthy text-book. Practical and clear."— ^'atwre.

Seventh Edition, Revised and Enlarged. With many Exam. Questions. 3s. 6d.
PRACTICAL ELEMENTARY MANUAL OP

MAGNETISM AND ELECTRICITY.

**The work has very high qualities, which may be condensed into the one word
" cle&T.'*— Science and Art.

LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND.

MECHANICAL ENGINEERING

FOR BEGINNERS.

R. S. M'LAREN.

litb Vlumetou0 5llu6ttation6.

OF THE

UNIVERSITY

LONDON:

CHARLES GRIFFIN & COMPANY, LIMITED;

EXETER STREET, STRAND.

1908.

eafjiAi

PREFACE.

Of books dealing with the various branches of Mechanical
Engineering there is an immense choice. Reliable text-
books can be found dealing with almost any subject upon
which an engineer can desire information ; but when
asked by a beginner, say by a youth whose friends have
just placed him as an apprentice or pupil with a firm of
mechanical engineers, to recommend an inexpensive and
up-to-date book on engineering, one finds some diflSculty
in making a selection.

To the author it appears that what a beginner really
needs is a book which, while giving in broad outlines
the information it is necessary to possess concerning the
ordinary branches of mechanical engineering, yet shall
go suflSciently into detail to enable him to make all the
calculations likely to be required during the earlier stages
of his career.

It has been the aim of the author in the following
pages to state in clear language some of the elementary
facts connected with mechanical engineering, and to show
how the simple calculations which have to be made from
time to time by every engineer and draughtsman can be
performed.

Theory is introduced in places where its bearing on
practice can be seen and understood. For instance, the
theory of raising steam is dealt with after the reader has
been introduced to the various types of boiler in use and
has learnt something of the merits and demerits of each.
Again a somewhat important law of Joule is not stated

203813

Vi PBBFACB.

until the Chapter on Steam Turbines is reached, when the
student is able to realise the importance of the law and
its application to the De Laval turbine.

The formulae given are of the simplest character and
can be worked out by a beginner having no knowledge
of algebra or mathematics, a knowledge of decimals only
is assumed

No attempt is made to describe mechanical processes,
such as turning, boring, planing, or iron and brass
founding, as the beginner will be able to acquire a know-
ledge of these processes in his passage through the shops.
It is the author's aim to give information which, unless
acquired by experience, can only be obtained by reading
a considerable number of books dealing with each subject
separately!

In actual work many practical questions, such, for
instance, as to the speed of a centrifugal pump, or of a
high-speed engine, or the heating surface of a given
Lancashire boiler, frequently arise. These are usually
settled approximately by referring to the makers' cata-
logues, such catalogues, however, are not often accessible
to the beginner, and the author hopes that the information
given in this book, much of which he has found useful
in his passage through the shops, drawing oflBce, and at
the directors' table, may prove of some service to others
who are just beginning their career.

R. S. MXAREN.

January^ 1908.

CONTENTS.

CHAPTER I.
Materials.

Wrought Iron — Cast Iron — Malleable Iron — Steel — Aluminium —
Copper — Gunmetal — Bronzes and Alloys — Testing Materi-
als—Stress — Strain — Elastic Limit—Reduction of Area —
Impact Tests — Alternate Bending Tests — Summary of
Weights and Strengths — Effect of Temperatures on
Strength— Fatigue of Materials — Factors of Safety —
Typical Specifications — Cost of Materials, , . . . 1-18

CHAPTER II.
Bolts and Nuts, Studs, Set Screws.

Bolts and Nuts — Studs — Set Screws — Locking Devices — Lock
Nuts — Grover Washer — Helecoid Nuts — Castle Nuts —
Steady Pins — Lewis Bolts — Foundation Bolts — Table of
Whitworth, and Gas Threads — Rivets — Diameter and
Pitch of Rivets for Various Joints, 19-26

CHAPTER III.

Boilers.

Boilers — Cornish and Lancashire — Galloway — Economic — Loco-
motive — Marine — Dry Back — Vertical — Dangers of the
Shell Boiler — Grooving, Pitting, and Wasting — Water-
tube Boilers — Babcock — Niclausse — Belleville —Stirling —
Thornycroft — Yarrow — Strength of Boilers — Steam Rais-
ing — Saturated and Superheated Steam — Evaporation —
B.T U. contained in Coal — Rate of Combustion — Draught
of Chimneys — Area of Chimneys — Ratio of Grate Area to
Heating Surface — Evaporation per foot of Heating Sur-
face — Proper Combustion of Coal — Transmission of Heat
through Boiler Plates — Feed- water and Boiler Composi-
tions— Testing Boilers, 27-66

PAGES

VUl CONTENTS.

CHAPTER IV.
Steam-Bauing: Aooeflsories.

Pumps — Duplex — ^Weir — Deane — Flywheel Pumps — Injec tors —
Feed-water Heaters and Economisers — Thermal Storage
System — Superheaters — Mechanical Stokers — Bennis—
vicars — Underfeed and Chain Grate — Howden's Forced
Draught — Meldnim*s Blower — Coal Conveying Plant —
Oil Filters 67-70

CHAPTER V.
Steam Pipes and Valves.

Material — Arrangement — Expansion — Size of Pipes — Flow of
Steam in Pipes — Radiation from Covered and Uncovered
Pipes — Strength of Pipes — Size of Flanges — Water Hammer
— Steam Traps — Bucket and Expansion — Exhaust Pipes —
6top Valves — Hopkinson - Ferranti Valve — Isolating
Valves, 71-88

CHAPTER VI.
The Steam Engine.

Action Explained — Lap— Lead - D-Slide Valve — Piston Valve —
Balanced Slide Valve— Reversing Gear— LH.P.— B.H. P.—
Method of Calculating Horse-power — Hyperbolic Curve —
Reason why Single Cylinder Engine Uneconomical —
Initial Condensation — Compound- and Triple-expansion
Engines — Consumption of — Corliss, Willans, Belliss and
Browett-Lindley Engines — Merits and Demerits of various
Types — Steam Jacketing— Calculating H.P. of Compound
Engines — Testing Engines — Brakes — ^Indicating an Engine
and Method of Working out Diagrams, .... 89-127

CHAPTER VII.

The Steam Engine.

Flywheel Calculations — Stored Energy — Stress in Rim — Throttle
and Expansion Governors — Proportion of Engine Parts
and Stresses — Engine Packings — Hemp, Asbestos, and
Metallic —Piston Rings and Springs — Efficiency of Engines
— Zeuner Diagram, 129-147

CONTENTS. IX

CHAPTER VIII.
Power Transmission.

PAQES

Rules for Power Transmitted by Belts — Speed of Belts— Com-
pounding Belts — Pulleys — Balancing and Convexity of
ruUeys — Speed of Driving and Driven Shafts — Thickness
of Belts for Small Pulleys — Fast and Loose Pulleys —
Ropes, Rules for Power Transmitted by — Speed of Ropes
— Size of Rope Pulleys — Distance Apart of Pulleys —
Clutches for Rope Pulleys — Loss of Power in Transmission
— Shafting, Rules for Power Transmitted by — Bearings
—Gearing — Toothed Wheels — Helical Wheels — Worm
Wheels — Skew Wheels — Raw- hide Pinions— Hans Rey-
nolds* Silent Chain -Rules for Power Transmitted by
Gearing, 149-163

CHAPTER IX.
Condensing Plant.

Object of Condensing Plant — Jet Condenser — Amount of Cooling
Water Required — Air Pumps — Surface Condenser — Extent
of Cooling Surface — Amount of Cooling Water Required —
Capacity of Air Pump — Temperatures and Corresponding
Vacua— Dry and Wet Air Pumps — Vacuum Augmenter —
Corrosion in Condenser Tubes — Edwards' Air Pump —
Evaporative Condensers — Ejector Condenser— Barometric
Condenser— Cooling Towers, 165-176

CHAPTER X.

The Steam Turbine.

Parsons Turbine— Willans-Parsons Turbine— Brush-Parsons Tur-
bine — Speeds and Outputs— De Laval Turbine — Curtis
Turbine— Rateau Turbine — Westinghouse Turbine — Zoelly
Turbine— General Remarks, 177-192

CHAPTER XL

Electrical Chapter.

Production of Current — Magnetic Field — E. M. P. — Simple
Dynamo — Volts, Amperes, Ohms, and Watts — Electrical
Horse-power — Power and Current Required for Lamps —
Reason for High Voltages — Alternators — Transformers —
Single and Multiphase Currents — Series- wound. Compound-
wound, and Shunt- wound Dynamos — Rotary Converters —
Motor Generators— ^Primary Batteries and Accumulators —
Summary of Electrical Terms— Table of Conductors, . 193-209

X CONTENTS.

CHAPTER XII.
HTdraulic Machineiy.

PAOBS

Hydraulic Press and Hand Pump —Hydraulic Accumulator-
Pressures Used by Hydraulic Engineers — Hydraulic
Riveters, Fixed and Portable — ^Flanging Press — Lifts and
Cranes -Jiggers— Jack — ^Water Wheels — Overshot, Under-
shot, and Breast Wheels — Turbines — Parallel Flow — Radial
Flow — Mixed Flow — Advantages and Disadvantages of
each — Impulse Wheels — Power obtained from Falling
Water — Suction Tube — Governing. — Bearings — Niagara
Falls Turbines — Falls of Foyers Turbines — Pumps- Cen-
trifugal — Multiple -Stage Pumps — Sizes and Speeds —
Reciprocating Pumps — Pumping Hot Water — Valves —
Pulsometer Pump — Hydraulic Ram — Materials for Pump-
ing Various Liquids — Flow of Water in Pipes and Useful
Memoranda, 211-237

CHAPTER XIII.

Gas and Oil Engines.

Otto Cycle — Description of 10-H.P. Engine— Methods of Ignition
— Exhausting and Scavenging— Drawing in Mixture — Com-
pression — Exceptions to Otto Cycle — Kiirting Engine —
Amount of Gas Consumed and B.T.U. Contained in Gas —
Horse-power of Gas Engines — Temperatures and Pressures
Reached — Speeds — Thermal Efl&ciency — Distribution of
Heat — Suction Gas Plant — Amount of Gas Made and
Water Required — Starting Gas Engines — Comparative
Merits of Steam and Gas Engines— Oil Engines — Con-
sumption of Oil and Petrol, 239-254

CHAPTER XIV.

Strengrth of Beams and Useful Information.

Strength of Beams having Top and Bottom Flanges — Strength of
Rectangular Beams — Table of Rolled-steel Joists — Ascer-
taining Stresses by Graphic Methods — Calculating Power
Transmitted by Screws, Levers, and Wedges — ^Thermo-
meter Scales — To Divide a Straight Line into a Number
of Equal Parts — Decimal Equivalents of an Inch with
Areas and Circumferences— Table of Areas and Circum-
ferences — Table of Squares and Cubes — Weights and
Measures with Metrical Equivalents, .... 255-269

CHAPTER XV.
Conclusion, 271-276

Index, 277

LIST OF ILLUSTEATIONS.

Fia& PAGE

1. Incorreqt method of ribbing a flat plate, . . . .11

2. Correct „ „ „ .... 11

3. Bolt and nut 19

4. SM, 19

5. Sei screw, 19

6. Lock nuts, 20

7. Grover washer, 20

8. Castle nut, 20

9. Heliooidnut, 20

10. Set screw and steady pin 20

11. Lewis bolt, 20

12. Foundation bolt and anchor plate, 21

13, 14. Rivets, before and after closing, 24

15. Lancashire boiler, 28

16. „ front view with setting, .... 29

17. Cornish boiler, front view, ♦ , 32

18. Locomotive boiler, 34

19. Marine ,, 35

19a. "Blake" „ 36

20. Babcock „ 38

21. Worthington Duplex pump, 67

21a. Weir feed pump, 59

22. Cameron feed pump, 60

23. Diagram showing action of injector, 62

24. Modern injector, 63

25. Feed-water heater, 64

26. Underfeed stoker, 68

27. Dangerous pipe arrangement, 72

28. Correct position of stop valve on boiler, .... 73

29. Incorrect ,, „ „ .... 73

30. Stop- valve passage obstructed by water, .... 77

31. Bucket ^team trap, 8.3

32. Expansion ,, . 83

33. Screw-down stop valve, 85

34. Straight- through sate stop valve, ..... 86

35. Vertical single-cyunder steam engine, 90

36. Cylinder with piston valve, 92

37. Reversing gear, 92

37a. Setting eccentrics, . 92

38. L.P. cylinder with balanced slide valve, ... .93
39, 40, 41. Hyperbolic curves, , , .99

42, Corliss gear, 105

43. L.P. cylinder with Corliss valves, 105

44, Drop-valve engine, , . 107

44a. Van den Kerchove en^ne, 107

45. Willans triple-expansion engine, 109

XU LIST OF ILLUSTEATIONS,

FI08. PAax

46. Crosby indicator 121

46a. „ „ for superheated steam, . . .121

47. H.P. Indicator diagram, 124

48. L.P. „ „ 124

49. Metallic packing, 141

50, 51, 52. RamsDottom piston rings, 143

63, 54, 55. Willans piston rings, 143

56, 57. Mudd piston rings 143

58. Zeuner diagram 146

59. ,, with piston and cylinder, .... 146
60, 61. Worm and wheel, 160

62. Hans Reynolds' chain, 162

63. Jet condenser, 166

64. Air pump 167

65. Surface condenser, 168

66. Condenser tube end and ferrule, 169

67, 68. Edwards' air pump, 172

69. Willans-Parsons steam turbine, 178

70. Turbine blading, 179

71. „ baffle rings 179

72. Method of strengthening Parson's blades, . . . .179

73. Bucket wheel of De Laval turbine, 183

74. De Laval turbine with gearing, 184

75. Curtis turbine blading, 186

76. „ 187

77. Horse-shoe magnet, 194

77a. „ „ 194

78. Direct current 12-pole d3mamo, 199

79. Alternator, 201

80, 81, 82, 83. Alternating-current diagrams, 202

84. ,, ,, „ ..... 203

85. ,, „ generator 204

86. Hydraulic press, 212

87. Portable hydraulic riveter, 212

88. Hydraulic U leather, 212

89. „ accumulator 213

90. „ riveter 216

91. . ,, flanging press, 218

92. ,, punching machine, 218

93. „ jigger, 219

94. „ jack 219

95. Water turbine, Jonval type 222

96. y^ radial inward-flow type, . . .223

97. „ „ outward-flow type, .... 224

98. Centrifugal pump, 229

99. Pulsometer ,, 233

100. Gas engine, 240

101. Korting gas engine, 243

102. Suction gas producer 248

103. Homsby oil engine, 253

104, 105, 106. Graphic methods of computing streues, . . .260

MECHANICAL ENGINEERING

FOR BEGINNERS.

CHAPTER I.
MATERIALS.

The materials chiefly used in mechanical engineering are —
wrought iron, cast iron, steel, aluminium, gunmetal, and
kindred alloys. Iron may, for practical purposes, be divided
into two distinct classes — viz., wrought and cast — and although
the difference between them lies principally in the fact that
_^ there is less carbon and silicon in wrought iron than in cast
iron, yet in practice the distinction between the two forms of
iron is so marked that fchey might almost be two distinct metals.
Wrought Iron is fibrous, can be bent (or given a permanent
set) without breaking; it can be sheared and punched; when
hot it can be worked under the hammer, and forged into various
shapes. Two pieces of this metal can be joined or welded to-
gether by heating them to what is known as welding heat (about
1,600* F.) and hammering the two parts together. The surface
of wrought iron can be made exceedingly hard, if required, by
case-hardening. This is effected by surrounding the metal with
cuttings of leather, horn, and bone dust, raising the whole to a
considerable heat, maintaining the heat for some hours, and then
slowly cooling. The carbon from these chippings enters into the
composition of the metal, and forms a surface closely allied to
steel. Case-hardening can also be effected by heating the metal
to a bright red and applying prussiate of potash, allowing to cool
slightly, and then plunging into cold water. This method is
quicker than the one previously mentioned, but is more liable
to cause distortion.

Wrought iron is used in cases where considerable strength is
necessary, and the required form is a simple one, such as can be
obtained by forging, welding, and machining the metal, stamping
out of the solid, or where the structure can be built up of plates,
Angle and Tee irons. During recent years wrought iron has

*J MECHANICAL ENOINEERINO FOR BEGINNERS.

been almost entirely superseded in mechanical engineers' works
by mild steel, but the former is still used in cases where it
requires to be welded, as in chains, tubes, eye-bolts, &c. In .
cases where shocks have to be withstood, wrought iron is superior
to mild steel, as the former has not the same tendency to crystal-
lise or lose its fibrous character. The coupling links of railway
waggons are always made of the best wrought iron.

Wrought iron and mild steel are brought into an engineer's
works in the following forms, viz.: — Hound, square, and flat
bars ; sheets or plates. Tee, Angle, and Channel iron. The
metal is formed into these shapes by being passed through rolls
when hot.

Wrought iron varies considerably in quality; its distinguishing
marks, in addition to the maker's brand, are— B, BB, and BBB ;
the letter B signifying the word "best."

Wrought iron of g(KKi quality will safely bear a stress of 5 tons
per square inch in simple tension — that is to say, a bar 1 inch
square will safely support any weight, up to 5 tons, suspended
from it. If, however, the load is likely to be a varying one, say
from to the maximum, then the bar should not be loaded to
more than 3^ tons per square inch. The breaking stress of the
best Yorkshire bars in simple tension is about 28 tons per square
inch, and of Staffordshire bars about 25 tons per square inch.
The elongation before fracture varies from 8 to 30 per cent,
on a test piece 10 inches long. Wrought iron will safely bear
a stress of 4 tons in compression — i.e., a block 1 inch square
will safely support a weight of 4 tons placed upon it. Wrought
iron will begin to crush, or give, under a pressure of 13 to
20 tons per square inch. If subjected alternately to tension
and compression, wrought iron should not be loaded to more
than 2 tons per square inch.

A cubic inch of wrought iron weighs approximately '28 lb.
The weight of any piece or structure of wrought iron is, there-
fore, easily ascertained by finding the number of cubic inches it
contains and multiplying by '28.

Wrought iron melts at a temperature of from 2,700° to
2,920" F.* It contains not more than 2 per cent of carbon.

Cast Iron is granular and of a brittle nature ; it cannot be
bent, sheared, or punched, and is liable to break suddenly if too

*The melting point of metals is not easy to determine with great
accuracy. Mercury boils at 680** F., and, therefore, cannot be used.
Very high temperatures are usually determined by noting the resistance
caused to an electric current when passed through a platinum wire, the
latter being placed in a porcelain or quartz casing. The higher the
temperature the greater the resistance.

MATERIALS. 6

great stress is put upon it. Cast iron, however, can be run,
when in a molten state, into intricate shapes. Two pieces of
this metal, after working together for some time in sliding con-
tact, acquire very smooth surfaces, and the amount of friction
between them is less than with any other two similar pieces of
metal.

Cast iron is used in all cases where an intricate form is required
and sufficient metal can conveniently be given to withstand the
stresses likely to come upon it, and where corrosion can be
prevented by the friction of the parts or by other methods. Cast
iron is used for steam cylinders, bedplates, and frames of various
machines, for flywheels, large water pipes, railway chairs, and
for an enormous variety of purposes. Cast iron is brought into
an engineer's foundry in the form of pig iron : when required for
use it is broken into smaller pieces and melted down, together
with a certain proportion of old broken cast iron (scrap), when
this can be obtained, in a cupola.

The cupola consists of an upright wrought-iron casing lined
with fire-brick, fireclay, and a fire-resisting substance called
Ganister. The fire having been kindled, the cupola is charged
with coke and pig iron (broken) in alternate layers, and a blast
of air from a fan or blower is driven in, the iron melts and
trickles down through the coke to the bottom of the cupola. It
is then drawn off in a liquid state and poured into moulds made
to any desired shape in sand. While the first charge is being
drawn off and used, a further supply of coke and iron is thrown
in at an opening at the top of the cupola.

Pig iron is obtained in differing degrees of hardness and
strength ; these degrees are known by numbers. No. 1 is very
soft and easy to machine, but is deficient in strength, and is to a
certain extent porous. No. 2 is slightly harder than No. 1.
No. 3 pig is hard, tough, and close grained ; it is largely used
for good castings. Nos. 4, 5, and 6 are still harder and tougher
than the foregoing. The degree of hardness of old cast-iron
scrap depends upon the quality of the original metal and the
number of times it has been melted down. Scrap is cheaper
than pig iron, but castings, if made entirely from this source,
would be deficient in strength, very brittle, and extremely hard
to machine.

The cast-iron parts of machines that are subject to wear, such
as steam cylinders (in which the piston moves at a considerable
speed), should be made much harder than is necessary for those
parts which are not subject to wear, such as bedplates, &c. For
steam cylinders a certain proportion of No. 4, and even No. 5
pig, is added to a mixture of No, 3 pig and scrap iron. For

i MECHANICAL ENOIN BERING FOR BEGINNERS.

castings of an inferior kind it is customary to use a large pro-
portion of scrap and a very small proportion of No. 1 pig iron.
Every time, within certain limits, cast iron is remelted it becomes
harder and stronger, but more brittle. Small castings can be
made exceedingly hard by pouring the molten metal into iron
moulds or chills instead of into sand moulds. Castings treated
in this way are called " chilled."

Cast iron, if of really good metal, thoroughly sound, and free
from internal stresses set up by unequal contraction in coolings
will safely bear a stress of l| to H tons per square inch in
simple tension, but in cases where the stress varies with con-
siderable frequency from to maximum, the metal should not
be loaded to more than 1 or 1^ tons per square inch. In cases
where the metal is subjected to alternate tension and com-
pression, the working stress should not exceed ^ a ton per
square inch.

The ultimate or breaking tensile stress of cast iron varies
from 8 to 14 tons. It will safely bear a stress of 6 tons in
compression. The ultimate crushing stress is from 25 to 50
tons per square inch. A cubic inch weighs from •26 to -27 lb.
The melting point of cast iron is about 2,050* F. in the case
of No. 1 pig. Nos. 4 and 5 pig require a temperature of about
2,250" before melting. Cast iron contains from 3 to 3^ per cent,
of carbon and from li to 2J per cent, of silicon. Not more
than li per cent, of the carlK)n is combined with the iron, the
remainder is present in the form of graphite.

The transverse strength of cast iron is usually ascertained by
placing a bar, 2 inches deep by 1 inch wide, upon supports 36
inches apart, and loading it in the centre. A bar tested in this
way will break with a load of from 25 to 40 cwts., depending
upon the quality of the iron. In such a test the molecules in
the upper part of the bar are in compression, while those in the
lower part are in tension. A typical specification is given at the
end of the chapter.

Malleable Cast Iron is not so brittle as ordinary cast iron ^
it will bend slightly before breaking. Ordinary castings can be
made slightly malleable by surrounding them with a substance
which will extract some of the carbon, such as crushed red
hematite, or black oxide of iron (iron scale), and placing them
in an oven maintained at a temperature of 1,800' to 2,000" F.
during the day and at a dull red heat during the night for
several days and nights, the length of time depending upon the
size of the casting. If the castings are required to be very
malleable, they are made of the best Cumberland white or grey
pig iron. Such castings, before being treated, are extremely

MATERIALS. 5

brittle. Malleable castings are used in cases where shocks may
have to be withstood, and where the form renders it difficult
or expensive to make the article of wrought iron, and where
the expense of gunmetal or bronze has to be avoided.

Steel. — In steel there is not the same hard and fast division
between cast and wrought as there is between cast and wrought
iron, but for the purpose of classification finished steel may be
divided into three classes — viz., mild, cast, and tool steel.

Mild steel has many of the properties of wrought iron, but is
stronger. It can be bent, sheared, forged, and, if it has a very
small percentage of carbon, can be welded. Holes in steel
plates, unless the plates are very thin, should be drilled and not
punched, as punching injures the surrounding metal. Mild steel
can be cut or machined when cold by tool steel. Mild steel
usually enters an engineer's works in the form of round and
square bars, plates, and rough forgings. It is used in cases
where greater strength and hardness are required than are
obtained with wrought iron, but where in other respects the
latter would be used.

Mild steel is largely used for crank shafts, piston-rods, eccen-
tric-rods, boiler plates, cross-head pins, studs, rivets, &c. Steel
containing a small proportion of nickel is stronger than ordinary
mild steel, and does not rust so easily, but is more difficult to
machine. Mild steel will safely bear a stress of about 6 tons in
simple tension or in compression, or 4 tons when the load varies
from to maximum, but when subject alternately to tension and
compression, as in a piston-rod of a double-acting engine, 2J tons
per square inch is a sufficient load if a long life is desired. The
breaking tensile stress of mild steel is about 30 tons, and the
elongation 20 to 35 per cent, on 10 inches. The torsional
strength of mild steel is dealt with in the chapter on transmis-
sion of power, and some remarks as to its ability to withstand
shock will be found later on. The weight of a cubic inch is
•288 lb. Mild steel contains from '1 to '5 per cent, of carbon.
It does not appreciably harden if heated and plunged in water.

Cast Steel is harder and stronger than cast iron, and will
bend before breaking ; in fact, the best qualities are improved
by being forged. Great care has to be exercised in making
steel castings, as the metal in cooling gives off a gas, which,
unless got rid of, honeycombs the casting with small holes.
Molten steel is not so fluid as iron, and does not fill the
cavities so completely as the latter. Messrs. Whitworth cast
some of their steel under pressure in order to overcome these
difficulties. .

Cast steel is used in cases where great strength is required,

6 MECHAXICAL ENGINEERING FOR BEGINNERS.

and where the form is only moderately intricate. It is used for
toothed wheels, hydraulic cylinders, high-pressure valve bodies,
for the arms of hydraulic riveters, &c. Steel castings, if sound,
will safely bear a stress of from 4 to 6 tons per square inch
in simple tension or compression. The breaking tensile stress
is from 20 to 30 tons, with an elongation of 15 to 25 per cent,
on 2 inches. Whitworth fluid-compressed steel has an ultimate
breaking strength of about 40 tons per square inch, with an
elongation of about 32 per cent. The weight of a cubic inch
is -288 lb. Steel castings usually contain about '251 per cent,
of carbon. The melting point is from 2,400" to 2,600' F.

Tool steel is much harder than mild steel, and is more difficult
to forge. Tool steel usually enters an engineer's works in square,
hexagonal, or round bars ; it is cut and forged to the required
shape and size, and is then hardened and tempered to make it
sufficiently hard to cut mild steel, cast iron, or wrought iron.

Hardening is effected by heating the piece of steel to a cherry
red (IjOOO' to 1,800° F.) and then plunging it into cold water
or oil. A piece of steel treated in this way only would be too
hard and brittle for many purposes. It is tempered by being
again heated to a temperature of between 430** and 570° F. and
then suddenly plunged into cold water. If the piece of steel is
of adequate size the hardening and tempering can be done at
one heating. The tool is heated to a cherry red, the part to
be hardened only is plunged into water and cooled; sufficient
heat then travels along the steel from the uncooled portion to
raise the temperature of the cooled part to the required degree,
when the whole piece is plunged in water. The temperature is
known by the colour of a film of oxide which appears on the
bright steel.

Self- hardening Tool Steel. — If a tool, after being hardened
and tempered, is reheated and allowed to cool slowly it loses
its temper and becomes soft. When a tool is used for cutting
hard materials at a high speed the tool becomes hot, and if the
heat reaches a certain point the tool loses its temper. It was
found, however, that by adding tungsten and a small percentage
of manganese and chromium the steel became self-hardening —
that is to say, it did not become soft when heated and allowed
to cool. Mushet steel was the best known example of this class
of steel.

High-speed Tool Steel. — A still further advance in steel used
for cutting purposes has been made in what is known as high-
speed tool steel. In it the percentages of tungsten and chromium
are increased, and that of the carbon diminished. The steel is
heated almost to melting point, then cooled by an air blast ; then

MATERIALS. 7

reheated to a dull red and again cooled by an air blast. In
hardening and. tempering these steels in the works, the maker's
directions, which are sent out with every bar, must be followed.
Quenching in water is forbidden.

The percentage of carbon in tool steel varies considerably. In
ordinary cutting tool steel there is about 1 per cent. ; in Mushet
steel about 2*15 per cent. ; in high speed tool steel about from
1*25 down to '6 per cent. The ultimate tensile strength of tool
steel is from 40 to 60 tons per square inch. The elongation is 5
to 12 per cent, on 2 inches.

As steel is often referred to as being of the Bessemer or open
hearth process, it may be well to state briefly what these
processes are. Steel is obtained from pig iron by removing a
large proportion of its carbon, and as much of the silicon,
phosphorus, and sulphur as possible. In the Bessemer process
this is effected by blowing air through the molten pig placed in
a converter ; as a result the silicon is first burnt out and forms
slag, which is removed ; the carbon then burns, making a long
flame of carbon monoxide ; when all the carbon is burnt the
flame ceases and a certain amount of Ferro-manganese or Spiegel-
eisen is added to give the required percentage of carbon to the
steel.

In the Siemens-Martin open hearth process, pig iron and
wrought-iron scrap are melted together and a proportion of
hematite added ; the air is not blown through the metal as in
the Bessemer process, but a mixture of air and producer gas are
burnt above the molten metal. As i^e pig iron is mixed with
scrap iron the amount of silicon and carbon is not so great as in
a corresponding quantity of pig iron alone, and what silicon and
carbon there is is got rid of by oxidation caused by the burning
gases, aided by the oxygen of the hematite. If the whole of the
carbon is removed a certain amount of ferro-manganese is added
as in the Bessemer process.

The Siemens open hearth process is similar to the Siemens-
Martin, but in the former pure pig iron and not pig and scrap is
used.

Briefly, it may be stated that the Bessemer process is chiefly
used for making high carbon steel, and the open hearth process
for mild steel.

One frequently reads of "acid" and "basic" steel; these
terms mean that the lining of the open hearth furnace or
converter was made either of an acid or basic material. The
lining of the furnace has a certain effect in removing impurities
from the steel.

Steel which contains phosphorus is brittle and is thought to

^ MECHANICAL ENGINEERING FOR BEGINNERS.

deteriorate in time, especially if subjected to stresses of an
alternating kind.

A small amount of nickel, as already stated, increases the
tenacity of steel. A small percentage, too, of vanadium has a
beneficial effect ; it appears to cause the carbon to distribute itself
more evenly, and adds to the ability of the steel to resist shock.

AliimJTiinm is a metal which is used in cases where lightness
is required ; it can be cast into intricate shapes. It is very soft,
but has a fair tensile strength, approximately that of cast iron.
Its ultimate breaking strength in simple tension is about 8 tons
per square inch, and may in practice be loaded to 1 ton per.
square inch. An alloy formed of aluminium and zinc is still
stronger. Aluminium bronze is dealt with under alloys, A
cubic inch weighs '09 lb. The melting point is about 1,200' F.

Copper is fibrous and very soft ; it can be bent, sheared, or
hammered into various shapes when cold. After hammering it
requires to be annealed — i.e., raised to a high temperature and
then suddenly cooled.* Copper is a good conductor of heat and
does not readily corrode. Two pieces of copper can be joined
together by brazing : brazing is effected thus — ^the two pieces to
be united are placed together, brazing metal in the form of
borings is placed over the joint and the whole is covered with
powdered borax j the seam is then held over a coke fire or
gas fiame until the brazing metal melts and unites the two
pieces of copper. Borax, which acts as a flux, is thrown on
during the operation. A joint which has been properly brazed
is as strong as the metal itself.

Copper, on account of its heat-conducting properties, is used
in the construction of locomotive fire boxes. It is also an
extremely good conductor of electricity and is largely used by
electrical engineers. Copper is also used for small pipes which
require to be bent cold.

Copper will safely bear a tensile stress of 1| tons per square
inch at temperatures below 300* F. ; above this temperature the
strength rapidly falls off. The breaking tensile stress of copper
bolts and plates at 300° F. is about 14 tons per square inch, with
an elongation of about 35 per cent, on 2 inches. At 600* F. the
strength falls to about 10 tons per square inch. The melting
point of copper is about 1,950° F. If the heat-conductivity of
gold is taken as 100, that of copper is about 87, while iron or
steel has a heat-conductivity of about 37 only. A cubic inch
weighs -322 lb.

Gimmetaly Bronzes, and Kindred Alloys. — Gunmetal is

* When steel is annealed it is raised to a high temperature and slowly
cooled.

MATERIALS. ^

stronger than cast iron and will bend slightly before breaking.
It is soft and easy to machine, can be cast into intricate
shapes, is not porous, and does not readily coiTode, but is ex-
pensive; it cannot be forged or welded. Gunmetal is used in
cases where comparatively small castings are required to be of
an intricate form and of moderate strength, also in cases where
corrosion has to be avoided. It is used largely for the internal
parts of steam and water valves, for cocks, and engine fittings.

Gunmetal is largely used for bearings in which steel shafts
revolve, as the friction between this alloy and steel is not
excessive, and gunmetal, being the softer, wears away before the
steel: when worn to too great an extent the "brasses" can be
replaced. Bearings that are subject to heavy and continuous
loads are usually lined with white metal or anti-friction metal.

Gunmetal is composed of copper, tin, and zinc ; the amount of
each is varied slightly according to the purpose for which the
alloy is required. The proportions adopted by the Admiralty
are — copper, 88 parts; tin, 10 parts; zinc, 2 parts. By adding
more tin the alloy is made harder. The component metals, when
the required casting is of small or medium size, are melted down
in plumbago pots or crucibles ; when a large casting is required
the metals are melted down in a furnace.

Gunmetal will safely bear a simple tensile stress of IJ to 2
tons per square inch. The breaking stress is from 10 to 16 tons,
with an elongation of 7 J to 10 per cent. The weight of a cubic
inch is -3 lb. The melting point is about 1,800° F.

Phosphor Bronze has all the properties of gunmetal, but is
stronger ; when hot it can be forged and rolled into rods. It is
composed of copper, tin, and phosphorus, the Admiralty propor-
tions being copper 83, tin 10, and phosphide of copper 7. Phos-
phor bronze is used for valve spindles, pump-rods (where steel
should not be used on account of corrosion), bearings, and for
small castings in cases where greater strength is required than
can be obtained by the use of gunmetal.

Phosphor-bronze rods and forgings will safely bear a tensile
stress of 4 to 5 tons per square inch, and castings from 2 to
3 tons. The breaking stress is from 10 to 30 tons, with an
elongation of 10 to 30 per cent, on 2 inches. The weight of a
cubic inch is '3 lb.

Manganese Bronze, Stone's Bronze, and Delta Metal
have all the advantages of and are stronger than phosphor
bronze ; in fact, they can be made stronger than mild steel, but
at the expense of ductility. All these bronzes contain a small
amount of manganese, iron, and other hardening materials ; the
exact proportions are, however, a trade secret. These bronzes

10 MECHANICAL ENGINEERING FOR BEGINNERS.

are largely used for pump-rods, valve spindles, ships' propellers,
and in cases where a strong non-corrodible rolled rod or casting
is required. Manganese bronze, Stone's bronze, and Delta metal
rods will safely hea.r a tensile stress of 5 to 7 tons per square
inch, and castings from 3 to 7 tons. The breaking tensile stress
of these rods varies from 29 to 35 tons, with an elongation of
10 to 32 per cent, on 2 inches.

Aluminium Bronze. — An exceedingly strong and ductile
bronze is made by a mixture of 90 parts of copper and 10 parts
of aluminium. The ultimate breaking stress of this alloy, when
cast, is about 30 tons per square inch in simple tension, with an
elongation of 20 per cent, on 2 inches. When rolled, a tensile
strength of 38 tons, with an elongation of 28*8 per cent, on
2 inches, has been obtained. If, however, the alloy is subjected
to a temperature of about 570** F., the strength falls off and the
alloy becomes brittle. At a temperature of 750" F. the alloy is
so brittle as to be of little use. The weight of a cubic inch is
•273 lb.

Muntz Metal is more ductile, but not quite so strong as the
bronzes just mentioned. This metal consists of from 60 to
6:i per cent, of copper and from 40 to 38 per cent, of zinc. It
is very suitable for condenser tubes, and for bolts which have to
resist the corrosive action of sea water. The safe tensile stress
for Muntz metal rods is about 5J tons per square inch, the
breaking stress about 26 tons, with an elongation of 45 per cent,
on 2 inches.

Brass is inferior in strength to, and more brittle than, the
above-mentioned alloys. It is composed of copper and zinc, with
a small proportion of tin. The safe tensile stress is about 1 ton,
or less, per square inch.

White Metal, sometimes called Babbit metal, from the name
of the metallurgist who first introduced it, is very soft, and has
valuable anti-friction properties — i.e., it remains cool when sub-
ject to rubbing contact under heavy pressure, and for this reason
it is largely employed for lining bearings. White metal is
composed principally of tin, with an addition of antimony and
copper. The Admiralty proportions are tin 85 to 89, copper 7 to
2, and antimony 8 to 9. A cheaper form of white metal is made
of lead and a small proportion of antimony, but is not considered
to be so good. White metal is usually bought in ingots from
firms who have made a study of the subject. The weight of
anti-friction metals made from tin and antimony is about '26 lb.
per cubic inch. The weight, when made of lead and antimony,
is about '4 lb. per cubic inch. With regard to the strength,
Professor Goodman has recently found that the ultimate

MATERIALS.

strength of some white metal, consisting of lead 90 parts and
antimony 10 parts, was about 3 J tons in tension and 7| tons
in compression.

In making castings of iron, steel, gunmetal, or of any other
alloy, it is essential for strength that sharp corners should be
avoided, for in cooling the crystals set in a direction at right
angles to the face of the casting, and every sharp corner consti-
tutes a source of danger. Any sudden increase, too, in the
thickness of the metal should be avoided. It has been found,
for example, that a flat plate strengthened by ribs running into
a common centre or small mass of metal, as shown by Fig. 1, is

Fig. 1. Fig. 2.

Incorrect and correct methods of ribbing a flat plate.

not so strong as when strengthened in the manner shown by
Fig. 2, where the cooling of the metal can take place more
evenly. A good radius should be given to the ribs as they meet
the central ring. Pulleys and small flywheels, in which the
section of the rim differs greatly from that of the arms, usually
have the latter cast in the form of an S. This form allows a
certain amount of give while the metal is cooling.

Testing Materials. — Until comparatively recent years it
was considered that if the tensile strength and percentage of
elongation of a certain metal, or alloy, was known, a judgment
could be formed as to whether such metal or alloy was, or was
not, suitable for a given purpose. It has, however, been found
that of two pieces of steel giving equally good results both as to
strength and elongation, one may be capable of withstanding
shock very much better than the other, also that one may be
able to withstand reversal of stress better than the other. Tests
to ascertain the behaviour of materials under shock and reversal
of stress are now frequently made, but before describing the
methods of making these tests it may be well to state what takes
place when a piece of metal is tested to destruction in an ordinary
testing machine, so that the beginner may have clear ideas as to
terms, such as stress, strain, elastic limit, yield point, reduction

12 MECHANICAL ENGINEERING FOR BEGINNERS.

of area, <fec. When a small rod of metal, or test piece, is put
into a testing machine and its ends pulled apaH, the force
applied produces " stress *' in the metal and the metal stretches ;
the amount by which the metal stretches, or its deformation, is
called "strain." Up to a certain point metal retains its elas-
ticity, so that if the load is removed the metal returns to its
original form. When the stress has reached a certain point the
strain or deformation increases out of proportion to the stress,
and the metal, provided it does not fracture like cast iron, takes
a permanent set; the point at which this occurs is called the
elastic limit or yield point.* Naturally the safe working stress
of the metal or alloy is well below the elastic limit. If still
further force is applied after the metal has reached its elastic
limit, the rod becomes permanently stretched, and its area
becomes reduced ; finally, when a certain stress is reached the
rod fractures. This stress is the ultimate or breaking stress of
the material, but, as the area of the rod has become reduced, the
ultimate breaking stress can be stated in two ways — either as so
many tons per square inch of original section, or as so many tons
per square inch at point of fracture. The former is the most
useful information for practical engineers, the latter for scientific
investigators into the properties of materials.

The amount by which the rod has stretched is called its
" percentage of elongation." Thus, if the working length of the
test piece is 2 inches, and it stretches | inch before fracture, its
elongation is 25 per cent. As the stretching is greater at the
point of fracture than in the other parts of the rod, the per-
centage of elongation on a long test piece is less than on a short
one, hence the length of the test piece should be given at the
same time as the percentage of elongation if the information is
to be of service.

Testing machines have been designed and are in use in many
Technical Colleges and Laboratories, which, by means of multi-
plying levers and pencil, draw automatically a diagram showing
the actual deformation that takes place in the metal when
subjected to gradually increasing stress. Such diagrams are
called stress-strain diagrams.

Impact Tests. — In order to ascertain whether a given metal
or alloy is suitable for resisting shock, an impact test should be
made. A very convenient and simple machine for this purpose
was designed a few years ago by Mr. Izod and is made by Messrs.
Avery. Izod's impact testing machine consists of a hammer
suspended at the end of a swinging rod. The sample piece of

* The true yield point occurs slightly later than the point of elastic
limit, but for ordinary purposes they may be looked upon as identical.

MATERIALS. 13

steel or alloy to be tested is of small rectangular section and is
nicked to a uniform depth at the place where fracture is desired;
the test piece is placed in a vice immediately under the hamnier,
the latter is then swung back to a given distance and released ;
in falling the hammer strikes the test piece and fractures it.
The distance traversed by the hammer (shown by a dial and
pointer), after fracturing the test piece, shows the resistance of
the latter. When the steel or alloy under test is brittle the
hammer travels much farther after fracture than when the
material is suitable for withstanding shocks. In this connection
it may be mentioned that if mild steel is heated up to about
1,330** F., and then quenched in oil, its ability to withstand shock
is enormously increased.

A simple testing machine for ascertaining the ability or
otherwise of metals to withstand alternate bending has recen,tly
been devised by Captain Sankey. The machine, which is made
by Messrs. Casella, consists of a fixed and a moving vice, the
latter being provided with a long handle. The piece of metal to
be tested is gripped by the fixed vice and bent by means of the
moving vice and handle. The moving vice is arranged so that
the pressure exerted in bending the test piece is transmitted
through springs, and by an arrangement of ratchet, dial, and
pencil, a line or rather arc, indicating the pressure exerted, is
drawn every time the metal is bent, so that both the number of
times the metal bends before fracture and the pressure exerted
each time are automatically recorded. The results of tests made
with this machine agree very closely with tests made with a
much more elaborate machine constructed by Professor Arnold.

Table i. gives a rough summary of the safe and ultimate
tensile, compression, and shearing stresses of various materials,
with the percentage of elongation, also the weight of a cubic inch.

Some tensile and compression tests, made by Mr. Izod, upon
four pieces of crucible steel are embodied in the table, as they
show clearly how the tensile strength of steel increases with an
increased- percentage of carbon, while the elongation falls off.
With regard to the bronzes enumerated, the higher tensile
strengths are only obtained at the expense of ductility — i.e., as
the strength increases the percentage of elongation falls off.

It may be noticed that while the ultimate shearing stress of
wrought iron and steel is less than the ultimate tensile stress ;
in the case of cast iron the ultimate shearing stress is greater
than the ultimate tensile stress. The figures, which are based
upon trials recently made, are confirmed by some tests* made by

* Communicated to the Institution of Mechanical Engineers, in con-
nection with a paper on " Shear," by Mr. E. G. Izod, 1905.

MBCHANICAL ENQINEKEINa FOE BEGINNEBS.

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MATERIALS.

Professor Goodman of Leeds, and given in Table ii. This table
is instructive, as showing the reduction in area which takes place
when certain metals and alloys are tested to destruction, and
how different some of the results appear when they are given in
terms of "tons per square inch of original section," and when
given in " tons per square inch at point of fracture."

TABLE II. — Tests made by Professor Goodman at Leeds.

Ultimate

TenaUe

Tensile

Eeduction

Shearing

Stress-

Stress at

Stress-

Oriffinal
Section.

Point

Area.

Original

of Fracture.

Section.

Tons

per sq. in.

Percentage.

per sq. in.

Cast iron, ....

10-9

Nil.

12-9

Gunmetal (soft and ductile), .

14-9

19-5

30-7

142

, , (hard and brittle), .

12-4

12-8

17-4

Moderately hard steel, .

48-0

49-8

7-4

Mild steel, ....

23-6

29-5

67-5

18-9

Wrought iron (soft),

21-7

27-4

49-8

17-4

,, (merchant),

22-6

24-7

24-6

24-5

Copper (annealed), .

14-8

23-7

650

110

„ (hard-drawn).

17-2

24-1

47-0

23-3

Aluminium, ....

8-8

12-7

45-5

5-6

.The tensile strength of steel and of most of the bronze alloys
can be increased by cold rolling — i.e., reducing the section of the
rod by passing it through rolls. The increased tenacity is, how-
ever, gained at the expense of ductility. Copper and iron wire,
when drawn down by successive operations, is much stronger
than when in the form of rod. Copper wire will bear a stress
of about 25 tons per square inch (as against about 14 tons in
the case of copper rods or plates) even after annealing ; before
annealing the wire has even greater tenacity.

Fatigue of Metals. — It has been found from test pieces cut
from material which has been subjected for some time to reversal
of stresses and to shock, that its percentage of elongation has
fallen off from the original figure. This falling off in ductility
and real strength is sometimes referred to as the " fatigue of the
material." Wohler found out over thirty years ago that frequent
reversal of stress — i.e., from tension to compression, or vice versd ;
or the frequent application and removal of stress of the same
kind — was sufficient to fracture a rod, even if the maximum
stress never exceeded one which was safe when the load was a
permanent or steady dead load.

16 MECHANICAL ENGINEERING FOB BEGINNERS.

Effect of Temperature on Metals and Alloys. — The

strength of steel and good wrought iron is not adversely
affected by temperatures up to 500' F., but above this point
the tenacity begins to fall off. Cast iron appears to be some-
what unreliable at temperatures below. 32' F., and its strength
begins to fall off at about 200' F. Brass and gunmetal are
unsuited for temperatures above 400' F., and should not be
used in valves and steam fittings when the steam is super-
heated. Special alloys suitable for fairly high temperatures
can be obtained. The strength of copper falls off at a tempera-
ture of about 300° F.

Young's Modulus of Elasticity. — ^The student will fre-
quently find references to Young's modulus. This modulus is
the ratio that a given stress per unit of section bears to a given
elongation within the elastic limit per unit of length. This co-
efficient of elasticity varies with the quality of the material ; for :
good wrought iron it is about 12,000, for mild steel about 13,000,
and for cast iron about 7,000, if tons per square inch are taken.
Thus, if a force of 1 5 tons should stretch a bar 800 inches long
and 1 square inch area to the extent of 1 inch, the material still
retaining its elasticity, then the coefficient of the material would
be 12,000.

Factors of Safety. — The factor of safety, or number by
which the breaking strength of a material should be divided to
give its safe strength, depends entirely upon the conditions
under which the material will be used. In building con-
struction where the load may be a permanent or dead one,
a factor of safety as low as 3 is sometimes taken for steel
work. In boiler work in this country a factor of safety of
about 5 is usually taken; thus, if the breaking strength of
the steel plate is 30 tons, the safe stress will be assumed to be
6 tons. In the case of iron castings where internal stresses may
be set up in cooling, and in which there may be small blow-
holes, the factor of safety taken is usually not less than 8.
In deciding upon the factor of safety, the results of Wohler's
investigations, previously referred to, must not be overlooked.
Professor Unwin, who is an authority on testing and on the
strength of materials, considers that Wohler's experiments show
roughly that if the safe stress of a steel bar under a steady
permanent load is called 3, then the safe stress for the same bar
under a load which is alternately removed and replaced, will
be 2 ; while if the bar is subject alternately to tension and
compression, the safe stress will be only 1.

Quality. — The following clauses dealing with the quality of
cast iron, mild steel, and cast steel are taken from actual

MATERIALS. 17

specifications issued by engineers within the last year or
two: —

" Cast Iron. — All cast iron to be of good close-grained quality,
free from cracks, flaws, blowholes, or chilled spots. All cast-
ings under working stresses to be of metal to stand the following
tests, viz. : — A test bar 42 inches long by 2 inches by 1 inch in
section to be cast at the same time and from the same ladle.
The test bar when placed on its edge between supports 36 inches
apart to carry a load (gradually applied) in the centre, of
30 cwts., and to deflect under such load *2 inch. The harder
metal for the cylinders and liners to carry a load under the
same conditions of 40 cwts., and to deflect under such load '3 inch.

" Mild Steel. — The crank-shaft, piston-rods, and connecting-rods
to be of mild steel, having a tenacity of from 26 to 30 tons
per square inch,*and with an elongation of not less than 22 per
cent, in 10 inches before fracture. If required to do so by
the engineers, the contractors must send them ready prepared
specimens (of dimensions approved by them) cut from the
forgings for the purpose of being tested.

"Mild Steel Boiler Plates.— All the plates throughout each
boiler, and all the rivets and gusset stays are to be made of
open hearth mild steel. All the plates are to be placed with
their brands visible on the outside of the shell, or on the inside
of the flues, as the case may be. Every plate throughout the
boiler is to be tested at the expense of the contractor under this
specification by a longitudinal and transverse strip taken from
each plate. Every plate to be capable of standing a tensile
stress of somewhere between the limits of 27 tons and of 30 tons
per square inch of original sectional area, either lengthwise or
crosswise of the plates, with an extension of not less than 20 per
cent, in a length of 10 inches, which is to be the length of the
operative part of the test piece. In no case is the test piece to
have a less sectional area than half a square inch.

" Cast Steel. — All cast steel to be as free from blowholes as
practicable, and to be well annealed. All large blowholes to
be filled by electric welding, and castings so treated to be
annealed after welding. Test bars to be cast from the same
ladle at the same time as the bulk of the castings, and to give
a tensile strength of 27 to 30 tons per square inch, and an
elongation in 3-inch test bars of not less than 20 per cent,
before fracture.

"Manganese Bronze. — All manganese bronze used for bolts,
nuts, studs, &c., to be of high tensile strength, and test bars to
show a tensile strength of 25 to 28 tons per square inch, and an
elongation of not less than 20 per cent, before fracture."

MECHANICAL ENGINEERING FOR BEGINNERS.

Boiler makers, in ordering steel plates from the makers,
usually specify that a strip 2 inches broad and 10 inches long,
sheared from the plate and heated to redness, then cooled in
water at 80* F., must stand bending until the inside radius of
the curve is one and arhalf times the thickness of the plate. The
plates must stand this bending without showing any signs of
fracture.

Table iii., giving the approximate cost of the material with
which an engineer has to deal, may prove useful to a beginner.

TABLE III. — Approximate Cost op Materials.

Iron castings, plain and fairly heavy, .
,, intricate ,,

Pig iron (Scotch),

Wrought iron, i

or ; Plates, angles, and bars.

Mild steel, ^
Steel castings,
Aluminium ingots,
Gunmetal castings,
Phosphor bronze rods.
Manganese , ,
Stone's , ,

Copper ingots for melting.
Tin

Zinc „ ,,

Lead ' ,, ,,

White metal , ,

N,B, — The wide margin given in the prices of tin and copper is due to
the fact that the price of these metals fluctuates very considerably.

£9

to £12 per tor

£12

„ £18

£2 15s.

„ £.3 10s. „

£6

, £8 10s.

£28

„ £45

£120

„ £130

lOd.

,, Is. 6d. per lb.

lOd.

»> '8, ,,

lid.

» 1 * ®* » »

lid.

„ Is.

£70

,, £115 per ton

£150

„ £200 „

£170

„ £195 „

£17

„ £22 „

£50

„ £200 „

CHAPTER II.
BOLTS AND NUTS, STUDS, SET SCREWS AND RIVETS.

Bolts and nuts, studs, set screws and rivets are made of
wrought iron or mild steel; the annexed illustrations clearly show
the difference between each. Bolts, studs, and set screws are
used for bolting together two pieces of metal which may require
subsequently to be taken apart. Bolts and nuts are used in all
cases where there is room for the bolt head. Studs are used
where there is not room for a bolt head, as shown by Fig. 4, or
where it is undesirable to make a hole right through both pieces
of metal to be fastened together. The objection to a stud is that
should it break off, it is difficult to extract the portion of the
stud which is screwed firmly into the metal. Even if the thread
only gets stripped off, or the stud becomes bent, it is a troublesome
matter to replace it. A set screw is used in cases where there
is not room for a bolt head, and where it is undesirable to have
a projecting stud when one portion of the joint has been removed.

i-^V^ ^

^ r^ nA>

Fig. 3.— Bolt and nut. Fig. 4.— Stud. Fig. 5.— Set screw.

In ca$es where bolts and nuts are subject to vibration, there
is a danger of the nuts working loose and coming off. Various
devices are employed to minimise this danger : the oldest and
most common one is to use two nuts and to lock them together.
The two nuts are screwed down hard, the inner nut is then
unscrewed for a fraction of a turn, the outer nut being screwed

MECHANICAL ENOINEERINO FOR BEGINNERS.

down at the same time. The nuts are thus pressed one against
the other and are locked; two spanners are required for the
operation. A split pin is usually put through the end of the
bolt to prevent any chance of the nuts coming off, should they
come unlocked.

Fig. 6.
Lock nuts.

Fig. 7.
Grover washer.

Fie. 8.
Castle nut.

Another device recently introduced is the Grover washer,
which consists of a split spring washer, as shown in Fig. 7. When
the nut is screwed down it compresses the washer, and thus the
threads of the nut are always pressed tightly against those of
the bolt. Grover washers of small size are of plain rectangular
section and not as shown by the illustration. In the Helicoid
nut (Fig. 9) the nut itself takes the form of a spring and grips
the bolt.

/ V^ ^

Fig. 9. -Helicoid nut.

Another device is the Castle nut (Fig. 8) ; this nut is provided
with saw cuts or narrow grooves ; a split pin is passed through
one of these grooves and through a hole in the bolt. If one of
the saw cuts does not happen to come opposite the hole in the
bolt, the nut is removed and a few touches with a file given to
the underside until one of the grooves comes into the desired
position.

Steady or Dowel Pins. — If two pieces of metal are fastened
together by set screws and are subject to lateral motion, such

Fig. 10. — Set screw and steady pin.

Fig. 11.— Lewis bolt.

BOLTS AND NUTS, STUDS, SET SCREWS AND RIVETS.

motion causes the sharp edges of the threads to cut into the
adjoining metal ; to overcome this, one or more steady or dowel
pins are usually put in as shown by Fig. 10. A steady pin is
merely a plain round piece of steel slightly tapered and driven
into a correspondingly tapered hole ; it effectually prevents any
lateral motion.

Fig. 1 1 shows a Lewis bolt, such as is sometimes used for
holding down machine-tools or small pieces of machinery to
concrete foundations. The Lewis bolts are put into holes large
enough to receive the largest part of the bolt ; the space round
the bolt is then run in with cement or molten lead. Large
pieces of machinery usually have long holding bolts provided
with square flat anchor plates at the lower end. In some cases
the bolt has a solid head under the anchor plate, in which case
the chase or square hole containing the bolt and anchor plate is
not run in with cement until the bed-plate has been placed in

^TP

Fig. 12.— Foundation bolt and anchor plate.

position. In other cases there is an oblong hole in the anchor
plate through which the bolt-head, also oblong, is dropped. The
anchor plate is provided with stops which prevent the boli>-head
making more than half a turn — i.e., until the oblong bolt-head is
at right angles to the oblong hole. A bolt with a head and plate
of this description can be withdrawn and replaced should it be
necessary to do so. The hole through the foundation is, of
course, left sufficiently large for the head to pass.

In other cases hand holes are provided in the foundations to
give access to the lower ends of the bolts, and a cotter or flat
piece of steel, as shown by Fig. 12, is placed through a slot in
the bolt, and serves as a head. If it is desired to replace the

MECHANICAL ENGINEERING FOR BEGINNERS.

bolt, the cotter is knocked out and the bolt drawn up through
the foundation. The end of the bolt passing through the anchor
plate is square. This gives increased section to compensate for
the slot, and prevents the bolt from turning while the nut is
being tightened up.

The following table, giving the number of threads per inch of
a Whitworth screw, and the diameter of the bolt at the bottom
of the thread, with the corresponding area, may be found
useful : —

TABLE IV.—Whitworth Threads.

Diameter of
Bolt.

Number of
Threads
per luch.

Diameter at

Bottom
of Thread.

Area at

Bottom of

Thread.

Thickness of
Head.

Thickness

of Nut =

Diameter of

Bolt.

Inches.

Sq. Inches.

Inches.

•186

•027

•219

•295

•068

•328

•392

•121

•437

•508

•202

•547

•622

•303

•656

•840

•553

•876

1^067

•894

1092

1-286

^•298

1312

1-493

1760

1531

1-715

2-309

1-75

2179

3-724

2-18

2-634

5439

2-62

3-573

10027

3-5

4-534

16146

4 5

5 489

23-65

6-25

The ordinary Whitworth thread is rather coarse, and cuts
into the metal to a considerable extent. Wrought-iron pipes
for steam, water, and gas are screwed with a much finer thread,
which is usually known as " gas thread." This thread is often
used for parts of machinery where a fine adjustment is required,
and where it is inadvisable to cut deeply into the metal. In
speaking of a ^-inch gas thread, a thread suitable for a pipe,
the internal diameter of which is i inch, is meant. The ex-
ternal diameter, the number of threads per inch, and the
diameter at the bottom of the thread, are given in Table v.

To ascertain what load a rod of wrought iron, say 1 inch in
diameter, screwed with Whitworth thread will safely carry, it is
necessary to take the area of the bolt at the smallest part— viz.,
at the tiottom of the thread. From Table iv. it will be seen

BOLTS AND NUTS, STUDS, SET SCREWS AND RIVETS.

TABLE v.— Gas Threads.

Internal

Number of

Diameter
Outside.

Diameter

Diameter of

Threads

at Bottom of

Pipe.

per Inch.

Thread.

Inches.

•382

-336

•518

•456

•656

-588

•825

•734

•902

•810

1-041

•949

1189

1097

1-309

1-192

1-492

1-375

1650

1623

1-745

1-628

1-882

1-766

2021

1^904

2116

2-000

2-347

2-231

that the area of a 1-inch bolt at the bottom of the thread is only
•553 of an inch. If, however, it is decided to use a gas thread,
so as not to cut so deeply into the metal, and we wish to know
the area at the bottom of the thread, we must refer to Table v.
From this it will be seen that the gas thread of a f pipe is
1^041 inches outside, so that a f gas thread would be used. The
diameter at the bottom of this thread is -949, as compared with
•840 in the case of the Whitworth thread.

In calculating the load a bolt will bear, it must not be over-
looked that a considerable stress may be imparted to the bolt by
the act of tightening up the nut, and for this reason a consider-
able margin must be allowed. It is not good practice to allow a
greater stress than 1 ton per square inch on bolts under f-inch
diameter, or li to 2 tons per square inch upon bolts above this
size. Bolts, the nuts of which require to be well tightened up,
should be not less than |-inch in diameter. With ordinary force
applied at the end of a spanner it is possible to break a f-inch
bolt, and to put an undue stress upon a J-inch bolt. Calculation
shows that, neglecting friction, a force of 10 lbs. applied at the
end of a spanner 8 inches long is sufficient to cause a stress of
53 tons per square inch on a §-inch bolt, and 22 tons per square
inch on a J-inch bolt. In actual practice, however, the friction
between the nut and the metal on which it presses absorbs some of
the force applied to the spanner. The friction between the threads
of the nut and bolt transmits a torsional stress to the latter.

MECHANICAL ENGINEERING FOR BEGINNERS.

As the diameter of a bolt increases, the area of metal increases
very rapidly — viz., as the square of the diameter, and the stress
per square inch due to tightening up falls off. In the case of a
1-inch bolt and a force of 20 lbs. applied at the end of a spanner,
10 inches long, a stress of about 8 tons per square inch is put on
the bolt. We see from the table that the area of a 1-inch bolt
is '553 inch at the bottom of the thread, and if we allow a stress
of about 1| tons per square inch, we find that the bolt will carry
•83 of a ton in addition to the stress put upon it by tightening
up the nut, or 8 '83 tons per square inch altogether. This, al-
though a fairly high stress, is well below the elastic limit of
wrought iron, and is only about one-third of the breaking stress
of the metal.

The most suitable number of bolts, their diameter, and pitch,
for circular flanges will be found in the table of pipe flanges,
prepared by the Engineering Standards Committee, given in
Chapter v. It will be noticed that bolts smaller than |-inch
in diameter are not used, even with the smallest flanges.

Bivets are only used for wrought iron and steel work, and in
cases where the two pieces of metal will not require to be taken
apart at any future time. Rivets, unless
of very small size, are heated before
being closed ; the contraction of the rivet
while cooling draws the plates tightly
together. Large rivets are usually
closed by means of hydraulic riveters ;
such riveting machines are described
in the chapter on hydraulic machinery.
The size of rivets and the distance
they aie spaced apart or pitched depends upon the thickness of
the plate and the kind of joint used. The following tables give
approximately the dimensions found in practice : —

TABLE VI.

tt3J^

PigH. 13 and 14.— Rivet
before and after closing.

SiNGLE-BIVETED LaP JoINT.

Thickness of

Diameter of

Piteh of

Plate.

Rivet.

Lap of Plates.

Inches.

2§

BOLTS AND NUTS, STUDS, SET SCREWS AND RIVETS.

DOUBLK-RIVETBD LaP JoINT.

Thickness of
Plate.

Diameter of
Rivet.

Pitch of
Eivet.

Lap of Plates.

Inches.

Double-riveted Butt Joints -Double Straps.

Thickness of
Plate.

Diameter of
Rivet.

Pitch of
Rivet.

Width of Butt
Straps.

Inches.

H
H

Inches.

H
34
34

4
44
51
54

Inches.
84

I'
10

12
13

Treble-riveted Butt Joints— Double Straps.

Thickness of
Plate.

Diameter of
EiTet.

Pitch of
Rivet.

Width of Butt
Straps.

Inches.

i
14

Inches.
H
it
Ifir

Inches.
6

Inches.
12
13i
18
19i
21
24

A lap joint, as its name implies, is one in which one plate
laps over the other. A butt joint is one in which the two

26 MECHANICAL ENGINEERING FOB BEGINNERS.

edges of the plate meet; separate straps or cover plates are
used on one or both sides of the plate.

The pitch referred to in the tables is the distance from centre
to centre of the rivets in the rows running parallel to the joint.
In double riveting the diagonal pitch — i.e,, the distance from
the centre of the rivet in one row to the centre of the rivet in
the next row, measured diagonally, should be not less than
•65 P + -35 D; where P = pitch of rivets, D = diameter of
rivets.

The thickness of conical rivet heads should not be less than
three-quarters of the diameter of the rivet. The strength of
riveted joints is dealt with in the chapter on boilers.

CHAPTER III.
BOILERS.

Boilers may be divided into two distinct classes — viz., the Shell
and the Water-tube. Boilers of the Cornish, Lancashire, loco-
motive, and kindred types are shell boilers — i.e., they consist of
a cylindrical shell containing the water and steam. The shell
contains combustion chambers in which the fuel is burnt, and
flues or tubes through which the gases pass; the combustion
chamber and tubes are surrounded by water.

In the water-tube class of boiler the water is contained in,
and circulated through, a large number of comparatively small
tubes; the tubes are connected either directly to, or through
headers with, drums containing steam and water. Amongst
the best known water-tube boilers are the Babcock, Stirling,
Thomycroft, Yarrow ; there are many others. We will first
consider boilers of the shell type, pointing out their good
features and the dangers connected with their use.

Lancashire Boiler. — Fig. 15 shows a Lancashire boiler in
section, and Fig. 16, a front view of the same boiler. This
type of boiler has been in use for a great many years, and has
many points in its favour. It is fairly inexpensive to construct,
and is sufficiently large to allow of a man getting inside to
inspect and to chip away any scale, should such be formed.
When steam has been raised it is easy to maintain it at a constant
pressure, even when sudden demands for large quantities of
steam are made upon it,, owing to the large volume of water
(ready to evaporate) and of steam which it contains. The
objections to this boiler are those which apply to all boilers
of the shell type, and are dealt with later. Apart from
these objections, all that can be urged against a boiler of the
Lancashire type is that it occupies a good deal of space, and
a considerable time is required to raise steam if the boiler is
allowed to get cold.

Fig. 15 represents a 30 feet by 8 feet Lancashire boiler
suitable for a pressure of 160 lbs. ; there are a few points
about it to which attention should be drawn.

In the first place, a true section through the centre of the
boiler would pass between the two flues, and the outside of one
flue only would be seen. In order to show the inside of the

MECHANICAL ENGINEERING FOR BEGINNERS.

BOILERS.

flue, a section following the dotted line A A, B B (Fig. 16), is
given. This conventional way of showing a Lancashire boiler
has some drawbacks — for instance, the gusset stays, which
prevent the ends from bulging out under the pressure of
steam, appear to go close up to the flue, while in reality there
is a space of 9 inches or 10 inches between the stay and the flue,
as will be seen from the end view. The object of this breathing
space, as it is called, is to allow the end plates to " give " slightly
when the flues expand or contract.

It will be noticed that while the shell consists of six rings of
rather wide plates, the flue is composed of a much larger number

V//'/^/
1//////

■////■ ^/. ^
/ / / // ///-;;

r////////M: > V: vv.^v/^!>; y /A ^ ;^

Fig. 16. — LaDcashire boiler, with brick setting.

of narrow rings. The reason for this is as follows : — The fewer
plates there are forming the shell the less riveting is required,
and a sounder and less expensive boiler is produced. As much
wider plates can be obtained now than was formerly the case,
the number of rings forming the shells of Lancashire and other
boilers has been considerably reduced. The width of the furnace
rings, when joined together by " Adamson " rings, is determined,
on the other hand, by the working pressure of the boiler, as the
strength of the flue to resist collapsing pressure is largely
dependent upon the number and distance apart of these rings.

30 MECHANICAL ENGINEERING FOB BEGINNERS.

In a boiler constructed for only 80 lbs. working pressure, the
furnace rings may be about 3 feet 6 inches wide, and the
Adamson rings be spaced this distance apart from one another ;
but for a boiler to work at a pressure of 160 lbs., the furnace
rings should only be 2 feet 3 inches wide ; while for a pressure
of 200 lbs. the rings should be still narrower — viz., about 2 feet
1 inch wide.

An enlarged sectional view of the Adamson ring is given in
the top right-hand comer of Fig. 15. It will be noticed that the
rivet heads are not exposed to the flames or gases, while the
central plate enables the joint to be caulked.

The figure shows four Galloway cross tubes in dotted lines
only, as the modem tendency is to dispense with them. These
tubes give increased heating surface, and for many years were
believed to improve the circulation of water in a boiler, but the
manner in which the tubes become coated with scale has made
some engineers sceptical as to whether the circulation through
them is as rapid a^ was thought. In any case, as soon as a
tube becomes coated with scale, its efficiency, from the point of
view of heating surface, rapidly falls ofil The tubes act very
efficiently as cross struts to support the flue, but by placing
Adamson rings closer together, as already described, cross tubes
as struts may be dispensed with. Leakage at the flanged joints
of these tubes is also sometimes experienced, and to avoid this
some boiler makers have welded in the cross tubes, but this
operation is not altogether an easy one, and, unless the welding
is very thoroughly done, it may lead to trouble.

The old-fashioned steam dome has been discarded for many
years on Lancashire and Cornish boilers. The reasons are two-
fold ; in the first place, the dome did not ensure dry steam passing
to the engine, and in the second it weakened the boiler con-
siderably. With regard to obtaining dry steam, it has been
found that if a large quantity of steam is collected from an
aperture placed over a small area of water, the rush of steam
carries with it small particles of water, and this is called
"priming." An anti-priming pipe, as shown by Fig. 15, is now
generally fitted ; this is merely a pipe with closed ends, having a
large number of slots or perforations in the upper half of its
circumference. This pipe collects steam evenly from a fairly
large area, and is more effective than the old-fashioned steam
dome.

The fitting shown at the extreme left of the boiler consists of
one spring loaded and one dead weight safety valve mounted on
a common seating. The spring safety valve is usually provided
with a lever, by moving which (by a chain or otherwise) the

BOILERS. 31

valve may be raised and steam blown off through a pipe fitted to
it. The dead weight safety valve is usually loaded to blow off
at 5 lbs. greater pressure than the spring valve, the steam
escaping to the boiler-house.

The fitting shown to the right of the stop valve and anti-
priming pipe is a high pressure and low water alarm safety valve.
If the level of the water falls too low the float sinks and opens
a small valve which may communicate with a whistle; in the
figure the float is apparently almost touching the flue, because
the figure does not show a true section ; in reality the float
can descend for a short distance between the two flues.
Surrounding the small valve, actuated by the floaty there is
another valve which acts as an ordinary safety valve should the
steam pressure rise too high.

The manhole shown to the right of the low water alarm is to
enable a man or boy to get inside the boiler for examination or
for cleaning : a strengthening ring is riveted round the shell, just
below the manhole.

The elbow at the bottom of the boiler is for blowing out any
sediment which may have accumulated. The elbow is usually
provided with a cock ; this cock is one of the most troublesome
fittings about a boiler, as it gets cut by the outrushing steam,
water, and dirt. Some users employ a cock next the boiler with
a valve beyond it ; by keeping the latter closed until the cock is
fully opened, the cutting action which is supposed to occur
while the cock is being opened is thus minimised. Any slight
leakage of the outer valve is immaterial.

The smaller fittings, such as the steam pressure gauge for
showing the pressure of steam, and the water gauge fittings for
showing the level of the water in the boiler, are not shown.
The latter are usually provided with thick glass shields or
protectors, or the glass tubes are partly surrounded by a brass
casing, so that in the event of the glass breaking, the pieces
may not strike the stoker in the face.

The following table, giving the sizes and approximate evapora-
tion of some Lancashire boilers of the usual sizes, may be found
useful : —

32 MECHANICAL ENGINEERING FOR BEGINNERS.

TABLE VII. — Lancashire Boiler.

Size of Boiler.

Diameter
of Flue.

Grate
Area.

Heating
Surface.

Approximate
Evaporation.

Ft. Ins. Ft.

Ins.

Ft. Ins.

Sq. Ft.

Lbs. per hour.

2U 0x6

2 7

607

4,260

22 X 6

2 7

563

4,700

24 X 6

2 7

620

6,200

24 X 7

2 9

670

5,600

26 X 7

2 9

730

6,000

28 X 7

2 9

790

6,500

30 X 7

2 9

850

7,000

30 X 7

890

7,500

28 X 8

3 2

920

7,500

30 X 8

3 2

960

8,000

30 X 8

3 4

1,010

8,500

-A^o^e.— The evaporation given for each boiler is "from and at 212** F.,"*
and is based on the assumption that between 20 and 25 lbs. of coal are
burnt per square foot of grate per hour. With a mechanical stoker more
coal than this can be burnt and greater evaporation obtained ; with hand
firing, unless the fireman is very capable, less fuel will probably be burnt
and a smaller evaporation obtained.

Cornish Boiler. — This boiler is similar to the Lancashire,
but is smaller, and has only one internal flue instead of two ; a
front view of this boiler is shown by Fig. 17. A Cornish boiler

Fig. 17. — Cornish boiler.

is suitable in cases where an evaporation of 1,000 to 4,000 lbs.
of water is required per hour. The following is a list of the sizes
usually made, with the approximate evaporation which niay be
expected ; —

* The explanation of this expression is given later.

BOILERS. 33

TABLE VIII.— Cornish Boiler.

Diameter

Heating

Approximate

Size of Boiler.

of Flue.

Surface.

Evaporation.

Ft. Ins. Ft. Ins.

Ft. Ins.

Sq. Ft.

Lbs. per hour.

14 X 5

2 8

220

1,250 to 1,700

16 X 6

2 8

250

1,400 „ 1,900

18 X 5

2 8

280

1,600 „ 2,100

20 X 6

2 8

310

1,750 „ 2,300

18 X 6

3 2

340

1,950 „ 2,600

20 X, 6

3 2

380

2,200 „ 2,900

22 X 6

3 2

400

2,400 „ 3,200

24 X 6

3 2

440

2,600 „ 3,500

26 X 6 6

3 3

530

3,000 ,, 4,000

In the majority of places where Cornish boilers are installed,
the stoker has duties other than those connected with the boiler;
under these circumstances a boiler which is somewhat large for
the output required is usually preferred, so that the stoking may
be done intermittently, without causing any serious fluctuations
in the steam pressure.

A Gkilloway boiler is a modification of the Lancashire ; the
two tubes are merged into one for a great part of their length, and
a large number of cross tubes are fitted. It is claimed that a
Galloway boiler will evaporate more water than a Lancashire
boiler of the same external dimensions, but, the form of the flue
not being truly cylindrical, it is not considered so suitable for
withstanding high pressures as the flue of a Lancashire boiler.

The Economic boiler (manufactured by Davey, Paxman &
Co.) is similar to a Cornish boiler, but is provided with a number
of small tubes running parallel to the main flue, through which
the hot gases return. This boiler occupies less space than the
Cornish, but more time requires to be spent upon it in cleaning
the tubes and keeping them tight. The boilers just described
are set in brickwork, flues being formed in it to bring the hot
gases back alongside the outside of the shell ; these gases then
return underneath the boiler to the chimney.

Fig. 18 shows a Locomotive boiler. This type of boiler is
employed in cases where a large amount of steam is required
from a comparatively small boiler, where portability is required,
and where brick setting is not admissible. It is used, as its
name implies, in locomotives, also for portable engines, traction
engines, road rollers, &c. ; it is occasionally used in steam in-
stallations of a temporary nature, and on some torpedo boats.
When used on a locomotive the boiler is capable of generating

MECHAKICAL ENOIKEEBINO FOB BROIKNER8.

BOILERS.

i??™*??

#?^

.':|o ooo ooo o oo

.::|oooo ooo ooo

:|o ooo oooo oo

aiiiiiiiiiiM|iiiiinni.

MECHANICAL ENQINEERINO FOR BEGINNERS.

more steam than when used in a stationary position ; this is due
partly to the powerful draught caused by the blast of exhaust
steam, and partly to the vibration which is believed to free
the bubbles of steam from the tubes.

In an English-built locomotive the fire-box is usually made of
copper. The tubes, which vary from If to 2 inches diameter,
are made of steel, iron, or brass. The locomotive boiler is
somewhat expensive to construct, owing to the large amount of
staying which the flat surfaces of the fire-box require; these
stays are frequently a source of trouble, owing to leakage and
breakages. A recent innovation on the Lancashire and Yorkshire
Railway is a mild steel corrugated fire-box which is said not to

Fig. 19a.—" Blake " boiler.

require stays. The fire-boxes of ordinary portable (loco, type)
boilers are usually made of the best Lowmoor wrought iron, or
of mild steel.

Fig. 19 shows a Marine boiler of the single-ended type,
which requires no brick setting; it is made of large diameter
and of small length, so as to utilise to the greatest advantage the
shape of the ship's hull. Marine boilers of the double-ended
type consist practically of two boilers as shown, placed back to
back, but with one combustion chamber common to both.

BOILERS. 37

A Dry Back boiler is similar to the single-ended marine,
but the back of the combustion chamber is formed of brick-
work.

The marine boiler is fairly satisfactory in use; its disadvantages
are— the great weight of the boiler when full of water, and the
length of time required to get up steam. The stays in the
combustion chamber sometimes give trouble, and the tubes
require to be expanded occasionally.

Vertical Boilers. — A large variety of vertical boilers
requiring no brickwork are made, and are suitable for cases
where a small quantity of steam only is required. The boiler
consists of a vertical shell, having an internal fire-box; there
are either cross tubes upon which the flames impinge, or a
number of small tubes through which the gases pass on their
way to the chimney. These boilers are compact and handy,
but are not economical in fuel, as a large percentage of the
heat generated passes away through the chimney.

One of the most economical boilers of this type is the Blake
Fig. 19a), made by The Blake Engineering Co., Ltd., Darlington,
'''his boiler has no flat surfaces and no stays.

Dangers of the Shell Boiler. — Unless boilers of the shell
type are examined periodically their use is attended with serious
risk of explosion, due to the plates becoming weakened, either by
grooving, pitting, or corrosion. Owners of such boilers usually
insure them against explosion, and the Boiler Insurance Co.
periodically sends specially qualified men to examine and report
as to the condition of the boilers. Grooving consists of the
formation of grooves in the plates of a boiler, usually near a
joint; the grooves are sometimes deep and narrow, and sometimes
wide and shallow. They are thought to be caused by undue
stresses coming upon a small area of the boiler plates through
unequal expansion and contraction. When the fire is first
lighted in a Cornish or Lancashire boiler the temperature of the
flues is raised, the outer shell remaining cool, and the expansion
of the flue, which in a 30-feet boiler may be as much as f inch,
tends to force out the end plates. The end plates, except in the
Thomson boiler, referred to later, are held in by gusset stays,
and thus, if the plates are thick and unyielding, undue stresses
may come upon portions of them.

It was found some years ago by Mr. Andrews, who made a
study of the corrosion of metals, that when two pieces of steel,
one of which had been strained to breaking point, and the other
subjected to very slight stress, were placed in a salt solution,
they formed a galvanic couple, and that the galvanic action, thus
set up, greatly increased the corrosion.

MECHANICAL ENGINEERING FOR BEGINNERS.

Pitting, or the formation of groups of small holes in a boiler
plate, is usually due, in the first place, to the presence of some
acid, or impurity in the feed water, which attacks the plates.
Wasting is the gradual decrease in thickness of the plates, due
to corrosion.

To avoid the stresses set up by the expansion of the flues, a
portion of the flue is sometimes made corrugated, as shown in
Fig. 19. It will be noticed in the illustration of the Lancashire
boiler that the end plates are stayed, or prevented from being

BOILERS. 39

forced outwards by the pressure of the steam, by gusset stays.
The latest development in Cornish and Lancashire boilers in this
country consists of dishing the ends of the boiler outwards in
the form of a saucer ; these ends being partly spherical, are said
not to require stays, and thus one of the sources of undue stress
in a boiler is removed. These dished boiler ends are, however,
stiff, and some engineers consider that corrugated flues should be
used when dished ends are employed. The dished boiler ends
have recently been introduced by Messrs. Thomson & Co.,
Wolverhampton.

The plates of shell boilers are almost invariably made of mild
steel. The rivet holes are all drilled, and the rivets are of mild
steel.

WATER-TUBE TYPE.

We will now consider boilers of the water-tube type. Fig. 20
shows the Babcock boiler. This boiler consists of a large
number of tubes, each about 4 inches external diameter, inclined
at an angle and expanded at each end into vertical rectanglar
tubes or headers ; these headers communicate with a drum which
is half full of water, the remainder of the drum forming a space
for steam. The water descends by the back headers, rises
through the inclined tubes, and passes up the front headers, thus
maintaining a very good circulation. The furnace is placed
under the front end of the tubes ; the gases are deflected by fire-
bricks, so that they pass completely over and under the whole
length of the tubes, the gases striking them at right angles. The
boiler, with the exception of the drum, is surrounded by brick-
work, but it is slung from joists carried on columns, so that it
is free to expand and contract.

The principal advantages of the Babcock boiler are as follows : —
Freedom from risk of explosion. — The steam and water drum is
of small diameter, and has no flat stayed surfaces; it is not
exposed to the fierce heat of the furnace, and is not subject to
severe stresses owing to unequal contraction and expansion.
The drum can therefore be made exceedingly strong for the
pressure it has to sustain. The tubes, which are of comparatively
small size, are capable of withstanding extremely high pressures;
even in the rare event of a tube failing, the result is not very
serious ; the tube may be rent, but unless the fire door should
happen to be open at the time (in which case the stoker might
be scalded), no harm is done, and the tube can be renewed at a
very small expense. Compared with the destruction brought
about by an explosion of a boiler of the shell type, the result of
a burst tube is insignificant.

40 MECHANICAL ENGINEERING FOR BEGINNERS.

Ability to raise steam quickly, — ^The water in a water-tube
boiler is very much subdivided, and as the circulation is
extremely good, steam can be raised very quickly; this is an
important point in electric generating stations in towns where a
fog may come on suddenly.

The boiler occupies a small space ; the heating surface is at
right angles to the path of the gases, and is thus in the best
position for extracting the heat from them. The thickness of
the metal through which the heat has to be transmitted is much
less than in a boiler of the shell type. For marine work the
Babcock boiler is slightly modified ; smaller tubes are used and
a metal casing, lined with specially light fire-bricks, takes the
place of brickwork setting. Boilers of the water-tube type have
a great advantage over shell boilers for war vessels, owing to
their ability to raise steam quickly. Their light weight, due
to the small amount of water held, is a point greatly in their
favour. The disadvantages of this type of boiler are dealt with
further on.

The overall length of a Babcock boiler is usually about 23 feet,
irrespective of its evaporative capacity, but shorter boilers are
made if required. The width may vary from 6 feet to 12 feet.
The evaporation ranges from 3,000 to 20,000 lbs. per hour per
boiler.

Kiclausse Boiler. — This boiler is somewhat similar to the
Babcock, but the tubes are connected to a header at one end
only, by means of coned joints, the other end of the tube being
closed. The circulation is obtained by placing one tube inside
another, and dividing the header by a diaphragm; the water
passes down the front portion of the header, flows through the
inner tube, returns through the outer tube, and passes up
through the back portion of the header. The advantage
claimed for this boiler is that the tubes can quickly be
removed for inspection and be replaced, while with most other
water-tube boilers a tube can only be removed by cutting it out,
and inserting a new one. The water in the Niclausse boiler
cannot periodically be blown off, as the lower ends of the
inclined tubes are closed ; this may be considered a disadvantage.
This boiler has had considerable success in France, but has
made little headway in this country.

The Belleville boiler has no headers back or front. Each
tube, which is of a zig-zag form, receives its water at the lower
end, and delivers the steam and water in a state of froth or foam
into a reservoir placed high up at the front end of the boiler,
and at right angles to the tubes. This boiler requires very
careful stoking.

BOILERS. 41

Stirling^ Thomycroft, and Yarrow boilers are all variants
of the same type of boiler. They have upper and lower drums
connected by small tubes, either inclined or bent. These boilers
have no headers such as are used in the Babcock and Niclausse
boilers. The tubes of the Yarrow boiler are straight, and so
give facilities for cleaning; those in the Thorny croft and Stirling
boilers are bent. All these boilers have proved successful ; the
Thomycroft and Yarrow are used chiefly for marine work, where
distilled water is used. There are many other water-tube boilers,
such as the Homsby, Climax, and others, but they vary chiefly
in design, and not in principle.

The disadvantage of the earlier water-tube boilers was their
lack of steam and water space, so that if an extra demand for
steam was made upon them, there was a tendency to prime, or,
in other words, for small particles of water to be carried off with
the steam. They also required more attention from the stoker
to keep the steam pressure and water level constant than a boiler
of the Lancashire or Scotch marine type. In the Babcock boiler
these disadvantages have been overcome by using a large steam
drum ; in the larger boilers of this make two drums are provided,
placed side by side.

Strength of Boilers. — Some simple calculations as to the
strength of boilers which a young engineer may be called
upon to make will now be explained.

The force exerted by the steam acting at right angles to the
surface of the water, and tending to burst or tear a boiler
longitudinally, is found by multiplying the internal diameter
of the boiler in inches by the steam pressure in pounds per
square inch. To resist this bursting force there is the thick-
ness of the metal plate on each side of the boiler, so that,
ignoring for the moment the question of riveting, the stress per
square inch in the metal plate forming the shell of the boiler can
be found thus —

D X P

-^-—f = stress;

where D = diameter of the boiler in inches.
P = pressure of the steam in lbs.
T = thickness of the plate in inches, or parts of an inch.

Example. — What is the stress in the metal plates (undrilled portion) of
a boiler 8 feet in diameter working at 150 lbs. pressure, the plates being
I inch, or '75 inch thick ? The calculation is

^^ = 9.600 lbs..
2 X -75

or about 4*3 tons per square inch.

42 MECHANICAL ENGINEERING FOR BEGINNERS.

The calculation does not, however, take into account the fact
that the plate is weakened by the holes which have been drilled
in it for the riveting. In order to find the stress upon the
weakest portion of the boiler shell, another factor is introduced,
and the formula reads —

D X P
2T X K^

where K = the percentage of strength of the joint, the method
of finding which will be given later.

Example. — ^What is the stress in the weakest part of the plate of a boiler
8 feet in diameter working at 150 lbs. pressure, the plates being { inch
thick, and the percentage of strength of the treble-riveted butt joint
79 per cent? The calculation is

96 X 150 ^12,152 lbs.,

2 X -75 X -79
or about 5*4 tons per square inch.

It is considered that the tensile stress upon mild steel plates
of Cornish or Lancashire boilers having butt joints at the longi-
tudinal seams should not exceed 1 2,500 lbs. per square inch, so
that the thickness of the boiler shell in the example just given
is about right. The figure, 12,500, is arrived at thus: — The
tensile strength of the boiler plate is probably about 28 tons,
and, allowing a factor of safety of about 5, we get the figure in
question.

To find the thickness of shell for any given steam pressure,
assuming we decide to allow a stress of 12,500 lbs. per square
inch, the formula would be transposed thus —

■^ To"R7^7i ^ = thickness.

2 X 12,500 X K

Example, — How thick should be the shell of a boiler 8 feet in diameter
for a working pressure of 200 lbs. ?

96 X 200 ^^ ^, . ,

2x12,500x79 ='^^^^^^^'

so that a plate 1 inch thick would be used. It must be noted that a
strength of 79 per cent, is only obtained with a treble-riveted butt joint ;
with a double-riveted butt joint the strength would only be about '75
that of the undrilled plate.

Apart from the effort of the st«am to burst the boiler longi-
tudinally, there is the effort to extend the boiler lengthwise,
or to pull it into two parts, owing to the pressure of steam on
the ends. The stress on the plates due to this effort is, how-
ever, only half the stress due to the pressure acting in the

BOILERS. 43

direction previously considered, so that if the plates are strong
enough to withstand the radial pressure, they are of ample
strength for the tension due to the pressure on the ends of
the boiler.

Dealing now with the strength of the boUer ends, it may be said
that the thickness of flat-stayed surfaces has been arrived at
from the results of experience, and that empirical formula have
been arranged to suit. The formula given by Mr. Hillier, chief
engineer of the National Boiler Insurance Co., is simple and
corresponds very nearly with actual practice ; it is as follows : —
p C X T^

where P = suitable working pressure in lbs. per square inch.
C = a constant given below.
T = thickness of plate in sixteenths of an inch.
S = area supported by one stay.

In Cornish or Lancashire boilers S = the area of the largest
circle which can be got between the gusset stays, and C = 220
approximately.

Example. — In the Laijicashire boiler illustrated, the end plates are
fl inch thick, and the largest circle which can be got between the gusset
stays is 16 inches (area 201). What may the working pressure of the boiler
be ? The calculation is

220 X 13-2 220 X 169 ,^.
201 ^' 201 = ^^^*
The answer is 186 lbs. per square inch.

If, instead of being flat, the ends are dished outwards, we can
find the stress per square inch in the plate by the following
formula : —

R X P :
2^1. = stress ;

where R = radius in inches of the dished end.

P = pressure of the steam in lbs. per square inch.
T = thickness of the plate in inches.

It is not advisable to allow so great a tensile stress in plates
which have been dished when red hot, as in plates which have
merely been rolled to form the shell, and a stress of 6,000 to
7,000 lbs. per sqtiare inch is considered about right. If we
decide to allow a stress of 6,000 lbs. per square inch, and wish
to know the thickness of a dished end, the above formula is
transposed thus —

R X P _ ^
6,000 X 2

44 MECHANICAL EKGIKEERING FOB BEGINNERS.

Example,— Kow thick should be the end plate of a boiler, S feet in
diameter, if the plate is dished out at a radius of 4 feet, allowing a stress
of 6,000 lbs., the boiler pressure being 185 lbs. ?

48 X 185 ^^ . . .
6,000x2= '74 thick.

If the end is flatter and dished out to a radius of 6 feet, the calculation
will be—

6,000 X 2

If the end is still flatter and is dished out to a radius of 8 feet, viz., the
same as the diameter of the boiler, the calculation will be —

96 X 185

6,000 X 2

= 1-48 thick.

The reason for multiplying the pressure by the radius, when
dealing with dished ends, instead of multiplying the pressure by
the diameter, as when dealing with cylinders, is because a sphere
is twice as strong as a cylinder, and the radius is, of course, only ,
half the diameter of a circle.

Strength of the Flues. — The pressui*e of steam on the flue
tends to make it collapse ; if the flue were truly cylindrical, and

D X P
the metal had no tendency to buckle, the formula -s m

would hold good, the metal being in compression ; but, as a fact,
the flues have a tendency to buckle, and the right thicknesses
and distance apart of the Adamson rings have been found by
experience.

Fairbairn's rule, which is sometimes quoted, was based on
experiments made many years ago with plain tubes, and the rule
does not hold good for flues having Adamson rings.

The Board of Trade formula, which is somewhat elaborate, is
given at the foot, but the following table will probably be of
greater service to the beginner. The table gives the distance
which the Adamson rings should be placed apart from one
another for various steam pressures, and the stress per square
inch which may be allowed upon the metal forming the flue.

w 1 • 9,900 X T /_ L + 12\
Working pressure = ^ -j^— ^^5 - -g^Y" j ;

where T = thickness of tube in inches.

L = distance between flanges in inches.
D = outside diameter of tube in inches.

Note, — L must not be greater than 120 T - 12.

BOILERS.

Steam

Distance AjMUt

Stress per Square Inch

Pressure.

of Rings.

on Metal.

Lbs.

Ft. Ins.

Lbs.

100

3,200 to 4,600

120

2 8

4,000 „ 5,000

160

2 3

4,750 „ 5,700

200

2 1

5,000 „ 6,000

Strength of Biveted Joints. — ^The strength of a riveted
joint, as compared with that of a solid plate, depends upon the
amount of metal which is left after drilling the rivet holes, and
upon the strength of the rivets to withstand shear. The strength
of the drilled plate is found thus —

P - D

where P = pitch of the rivet.

D = diameter of the hole drilled for the rivet.

Example, — What is the strength of a J-inch plate which has been drilled
with 1-inch holes at 4-inch pitch for a double-riveted butt joint ?

The answer is *75, so that the plate is only *75 times as strong as an
undrilled plate. If we take the case of a plate which has been driUed for
a single-riveted lap joint — ^viz., with 1-inch holes to 2J-inch pitch — we
shall see that it is still weaker, thus —
2-25 - 1 inch

—2^25 = ^^"^

so that the plate is only '555 times as strong as an undrilled plate.

The strength of rivets in ordinary practice is a little greater
than that of the drilled plates. The strength of rivets in a lap
joint is found by the following formula : —

A X N

^p rf\ = strength of rivets as compared with the drilled plate ;

where A = area of rivet.

N = number of rows of rivets.

P = pitch in inches.

T = thickness of plate in inches.-

-What is the strength of the rivets in a g-inch plate drilled

for I'inch rivets, 2i-inch pitch ?

•78 X 1

2-25 X -625 "" *^^^*
The rivets are, therefore, '557 times as strong as the undrilled plate.

In the case of butt joints with two straps, the rivets are in
double shear, and should be twice as strong as those in single

46 MECHANICAL EKGIKEERING FOR BEGINNERS.

shear, but as the Board of Trade reckon that a rivet which has
to be sheared through in two places is only 1*75 times as strong
as a rivet in single shear, this figure is usually taken, and the
formula for finding the strength of rivets in double shear is as
follows : —

A X N X 1-75

Example* — Wh&t is the strength of the riveta in a J-inch plate drilled
with 1-inch holes at 4 inch pitch for a double-riveted butt joint? The
calculation is

•786x2x1-75 ,^.
= 'loo.

4 X -876
The rivets in this case are *785 times the strength of the undrilled plate.

The size of rivets and the pitch usually adopted for various
joints and for various thicknesses of plate are given in Chapter ii.

PROPERTIES OF STEAM.

Steam. — Before discussing the evaporative power of boilers,
chimney draught, &c., a few words dealing with the raising of
steam may usefully be said. A clear idea as to what is meant
by the latent heat of steam is essential to anyone calling himself
an engineer.

If heat is applied to a boiler which is open to atmospheric
pressure, say 14*7 lbs. per square inch (the atmospheric pressure
varies, and is shown by the height of the barometer), a thermo-
meter placed in the water will show a rise of temperature
corresponding with the amount of heat put in until steam
begins to form. Under atmospheric pressure in normal con-
ditions this will take place at a temperature of 212" F., but
before 1 lb. of water can be evaporated at this pressure, a
very large further instalment of heat will require to be put
into the water; this heat will not be shown by the thermo-
meter. The heat, however, is there, and is called the latent
heat of steam.

It is necessary to know that the quantity of heat required to
raise the temperature of 1 lb. of pure water at its greatest
density by T F. is called a British thermal unit, or B.T.U.
What this thermal unit is equivalent to will be explained
later. Now, if there were no such thing as latent heat of
steam, the number of thermal units required to raise 1 lb, of
water from 32** to 212" and turn it into steam would be 180.
As a fact, the total heat which is required to raise I lb. of water

BOILERS. 47

from 32** to 212°, and turn it into steam at 14*7 lbs. pressure, is
1,146*1 thermal units. The diflference — viz. ,966*1 — between the
two sets of figures is the latent heat in 1 lb. of steam at a pres-
sure of 14*7 lbs. per square inch. The total heat of steam at
U*7 lbs. pressure is 1,146*1 B.T.U.

If, instead of taking the case of heat applied to water at
atmospheric pressure, we imagine heat applied to water under a
constant pressure of 120 lbs. per square inch absolute — by
absolute pressure we mean a pressure reckoned from a perfect
vacuum, and not above the atmospheric pressure— the thermo-
meter will rise steadily until it shows a temperature of 34F F.,.
at which temperature steam will begin to be formed; but to
raise the temperature of 1 lb. of water from 32° F., and turn it
into steam under a constant pressure of 120 lbs., 1,185*4 thermal
units will be absorbed.

If water is under a constant pressure of 200 lbs., and heat is
applied, the thermometer will rise to 381*7° before steam begins
to be formed, and 1,197*8 thermal units will be required to turn
1 lb. into steam. If, on the other hand, water is placed under a
vacuum and heat is applied, steam will begin to form at a tem-
perature much below 212° F. If a closed glass vessel, containing
water, is held in the hand, the upper portion of the vessel being
placed in communication with a very effective air pump, and an
extremely good vacuum is formed, the temperature of the- hand
is sufficient to cause the water to boil. The temperature at which
water boils is, therefore, dependent on the pressure upon it.

Table ix. gives the temperature of steam corresponding with
the pressure, also the total heat contained in 1 lb. of the steam.
The steam pressures are given in two columns, one giving the
gauge pressure, or pressure above the atmosphere ; the other the
absolute steam pressure.

Now, with regard to the British thermal unit, the energy

required to raise a weight of 1 lb. to a height of 1 foot is 1

foot-lb.; the energy required to raise 1 lb. 10 feet, or 10 lbs. to a

: height of 1 foot, is 10 foot-lbs. Joule found that 772 foot-lbs. of

; work were the equivalent of one thermal unit. That is to say,

\ if 772 foot-lbs. of work are expended on a lb. of water, by

violently agitating it with paddles, or by other means, the tem-

\ perature of the water is raised by V F, The work of 772*

! foot-lbs. is called the mechanical equivalent of heat.

If the amount of heat thus put into 1 lb. of water, could
i conveniently be utilised, it would be capable of lifting a weight
I of 1 lb. through a height of 772 feet (assuming no friction) ; after

i * More recent experiments show that this figure is rather low, and that
778 foot-lbs. i« a more accurate figure.

MECHANICAL ENGINEERING FOR BEGINNERS.

doing this work the water would be back again at its original
temperature.

TABLE IX.— Properties of

Saturated Steam.

• »i

.t,2

► a^

o .

> a!=5

"S .

^ajs

||1

a g

2*^1

a Q

J^^

Lbs. per
Sq. In.

Lbs. per
Sq.ln.

Degs.F.

B.T.U.

Cub. Ft.

Lbs.per
Sq.fo.

Lbs. per
Sq.ln.

Deg8.F.

B.T.U.

Cub. Ft.

102- 1

1112-5

330-36

100

327*9

1181-4

4-33

126-3

1119-7

172-08

105

333-1

1182-4

4-14

141-6

1124-6

117-52

110

334-6

1183-5

:s-97

153-1

11281

89-62

100

115

338-0

1184-4

3-80

162-3

1130-9

72-66

105

120

341-1

1185-4

3 66

170-2

1133-3

61-21

110

125

344-2

1186-4

3-51

176-9

1135-3

62-94

115

130

347-2

1187-6

3-38

182-9

1137-2

46-69

120

135

3501

1188-2

3-27

188-3

1138-8

41-79

125

140

3529

1189-0

316

193-3

1140-3

37-84

130

145

3556

1189-9

3-06

197-8

1141-7

34 62

135

160

358-3

11907

2-96

202-2

1143-0

31-88

140

165

3610

1191-5

2-87

205-9

1144-2

29-57

145

160

364-3

1192 2

2-79

209-6

1145-3

27-61

150

165

366

1192-9

2-71

14-7

212-0

1146-1

26-36

155

170

36S-2

1193-7

2-63

2131

1146-4

25-85

160

175

370-8

1194-4

2-66

216-3

1147-4

24-24

165

180

372-0

1195-1

2-49

219-6

1148-3

22-89

170

186

375-3

1195-8

2-43

222-4

1149-2

21-70

175

190

377 5

1196-5

2-37

225-3

1160-1

20-64

180

196

379-7

1197-2

2-31

228

1150-9

19-72

185

200

381-7

1197-8

2-26

2401

1154-6

15-99

190

205

383-8

1198-4

2-21

250-4

1157-8

13-46

195

210

385-8

1199-1

216

259-3

1160-5

11-65

200

215

387-8

1200-1

212

267-3

1162-9

10-27

206

220

389-9

1200-3

2-08

274-4

1165-1

9-18

236

250

401-1

1203-7

1-82

2810

1167-1

8-31

285

300

417-5

1208-7

1-53

287-1

1169

7-61

335

350

430-1

1212-6

1-32

292-7

1170-7

7 01

385

400

449-9

1217-1

116

298

1172-3

6-49

435

450

456-7

1220 7

105

302-9

1173-8

6-07

485

600

467-6

12-24*0

•94

307-6

1175-2

6-68

586

600

4870

1229-9

•80

312-0

1176-5

5-35

685

700

504-1

1235-1

-68

316-1

1177-9

5-05

785

800

519-5

1239-8

•60

320

1179-1

4-79

885

900

533-6

1244-2

•54

324-1

1180-3

4-55

985

1000

546-5

1248-1

•49

BOILERS. 4d

Steam in contact with water is called saturated steam ; this is
not the same thing as the steam being wet. Wet steam is steam
in which particles of water have become entrained through the
steam leaving the water at too great a velocity, or through the
temperature having' become lowered, and some of the steam
having turned back into water. Dry saturated steam is steam
which is free from particles of water. Superheated steam is
steam to which further heat has been added after it has been
formed. Such steam, if placed in contact with water, will absorb
a part of it, turning the water inte steam ; the temperature of
the steam falls slightly during the process. This addition of
heat to the steam, or superheating it, after it has been formed,
does not increase its pressure (apart from the pressure due to
increase of volume), and unlike latent heat, it is shown by a
thermometer.

Evaporation of a Boiler. — ^The evaporation of a boiler as
given by the makers is usually in terms of pounds of water evapo-
rated "from and at 212° F." This means that so many pounds
of water will be evaporated, if it is fed into the boiler at a
temperature of 212°, and is evaporated at atmospheric pressure.
If, however, the water is fed into the boiler at a lower
temperature, and is evaporated under a considerable pressure,
the evaporation of the boiler will be less than that given by the
maker. Thus, if water is fed into the boiler at a temperature of
60"* F., and is evaporated under a pressure of 120 lbs., the amount

of water evaporated will be only j^ that given by the makers.

If the water is fed in at 150** F., and evaporated at 120 lbs., the

evaporation will be about — • I^ ^©^ ^^ ^^ ^2° F., and evaporated

^^ 10-0

at 200 lbs. pressure, the evaporation will be ^-^ of that given.

The formula to enable one te find out what the actual evapora-
tion will be, if the evaporation "from and at" is known, is as
follows : —

B + 32 -

A =

966

where A = the factor by which the lbs. given as " from and at "
are divided.
B = the tetal heat oi the steam at the working pressure.
C = the temperature of the feed water.

Example, — ^If the makers say that a boiler will evaporate 1,200 lbs. of
water per hoar from and at 212^, how many lbs. will it evaporate if the
wcM^king pressure is to foe 120 lbs. (or 185 lbs. absolute) and the temperature

MECHANICAL BNGINEEBING FOR BEGINNERS.

of the feed is only 6(f F. ? The reader will see from Table ix. that the
total heat of steam at 120 lbs. pressure (above atmosphere) is 1,188 B.T.U.
The formula then, in actual figures, is

. 1,188 + 32-60 , .o
^ = 966 ^ ^-

The answer is 1*2, so that, if we divide the 1,200 lbs. given bv the
makers by 1*2, we get 1,000 lbs., which is the amount of water the boiler
will evaporate under the conditions given.

If the actual evaporation is known, and it is desired to
ascertain what the equivalent evaporation '^from and at 212'"'
would be, the number of lbs. actually evaporated is multiplied
by the factor obtained by the formula given above.

Coal and its Evaporative Power. — ^We have seen what a
certain number of B.T.U. are capable of doing in the way of
raising steam ; this information, however, is of but little practical
use to us, unless we know how many B.T.U. we can get from a
pound of coal. A pound of coal burnt in the laboratory gives
from 13,000 to 15,000 B.T.U. A pound of pure carbon gives
14,500 B.T.U., but coal contains other constituents than carbon,
such as hydrogen, oxygen, nitrogen, sulphur, and ash. The
B.T.U. values of 1 lb. of some of the best known coals, and of
petroleum, are approximately as follows : —

Powell DuflFryn,

Nixon's Navigation,

Newcastle,

Derbyshire,

Yorkshire, '

Scotch,

Coke,

Petroleum,

15,500
15,000
14,820

13,860

13,500

12,820

i 17,800

120,000

Steam coals are those which contain a large proportion of fixed
carbon and a small proportion of volatile constituents. They
burn without giving off much gas. A coal which fulfils these
conditions to the fullest extent is anthracite; the largest pro-
portion of coal is, however, bituminous or smoky coal.

Powell Duffryn is almost, but not quite, pure anthracite ; it
contains about 88*24 per cent, of carbon. Nixon's Navigation is
semi-anthracite. The others given above are bituminous ; they
contain from 75 to 83 per cent, of carbon, the remainder consisting
of various volatile constituents and ash. Coke contains from 86
to 88 per cent, of carbon.

Coals are sometimes divided into caking and non-caking coals;
the former soften when heated, and form a spongy mass; in
non-caking coals the particles remain separate and allow the air

BOILERS. 51

to pass between them. Cannel coal, which is very rich in volatile
constituents, is used for producing gas for lighting purposes.

The ash in Powell Duffryn coal is about 3 '26 per cent. In
Nixon's Navigation it is about 7 per cent., while in inferior coals
it may amount to 10 per cent, or more.

One lb. of coal, yielding 14,007 B.T.U., is theoretically capable
of evaporating 14*5 lbs. of water from and at 212" F. In actual
practice 1 lb. of coal evaporates between 8 and 10 lbs. of water.
Why, it may be asked, this great difference ? In the first place
there is the correction to be made for the fact that the feed may
not be so hot as 212**, and for the fact that the water will, of
course, be evaporated at a much higher pressure than that
corresponding with 212". The formula for making the necessary
correction has already been given. Then there is the loss due
to the heat which passes away in the gases to the chimney.
The temperature of these gases must not be lower than that of
the steam in the boiler, otherwise they would be harmful. The
losses from this source are probably from 12 to 15 per cent., and
may easily be greater. The other losses are those due to incom-
plete combustion, radiation, &c.

An economiser, which will be described later, will extract and
make use of some of the heat of the waste gases, and if the
boiler itself extracts 70 per cent, of the heat theoretically
contained in the coal it may be considered efficient.

When boilermakers say that a boiler will evaporate so many
lbs. of water per lb. of combustible, they mean that the weight
of the unconsumable ash must be deducted from the weight of
the coal burnt.

Bate of Combustion. — The number of pounds of coal which
can be burnt on each square foot of grate area depends on the kind
of coal and upon the draught. The draught given by a chimney
depends upon its height, assuming that its area is sufficiently
large to carry off the gases. Draught caused by the height of
the chimney alone is called natural draught*; if a fan is put inside

*The formula to enable the actual draught to be ascertained is as

follows:- D = H(^±-Ur-l:«-V

\A + 461/ \B + 46iy '

where H = height of chimney in feet.
A = temperature of external air.
B = temperature of gases in chimney.
D = draught.

Examine,— Wh&t will be the draught in a chimney, 100 feet high, when
the temperature outside is 60**, and inside 400**?

100 / 7'^ X. / 7-9 \ ^ .542
V60 + 461>> \400 + 46l7
The answer is -542 inch.

52 MECHANICAL ENGIKEERIKG FOB BEGINNERS.

the flue to increase the draught, the latter is called induced
draught; if a fan is employed to increase the pressure of air
in the stokehold of the furnace the draught is called forced
draught.

The intensity of the draught is spoken of as so many inches,
or parts of an inch, of water. This means that if a Cl-shaped
tube is partly filled with water, and one leg of the U is placed in
communication with the chimney, the other leg being open to
the atmosphere, the water in the leg connected to the chimney
rises above the level of the water in the other leg. If the
difference is ^ inch the draught is said to be equal to *25 inch of
water.

With a properly constructed chimney,* 100 feet high, and with
gases at a temperature of 400° above that of the atmosphere
outside, say 60** F., a draught of about *54 of an inch is obtained.
When fans are employed to give forced draught the draught is
usually between 1 and 4 inches. If the draught is forced to too
great an extent it is injurious to the boiler, owing to the intense
heat caused.

We have said that the amount of coal that can be burnt on
each square foot of grate area depends on the kind of coal and
on the draught. The following are approximately the amounts
of coal which can be burnt per square foot of grate, with a
draught of '5 inch, and with hand firing.

Coke, . . . .

11-12 lbs. per hour.

Nixon's Navigation,

. 18-20 „ „

Powell Duflryn, .

. 20-23 „ „

Bituminous coal, .

. 20-32 „ „

The nearer the coal approaches the qualities of anthracite the
greater is the draught required. Bituminous coal requires less

* A good empirical formula for determining the most suitable area for a
chimney is the following : —

where A =s area in square feet.

W = weight of coal burnt per hour.
H = height of chimney in feet.

Example, — What must be the area of a chimney, 160 feet high, for
boilers burning 2,000 lbs. of coal per hour ?

2.000 2.000

^ = iTTTo °^ ^ = r6^n2^4 = »<>-2 f-*-

The answer is 10*2 square feet.

A rule often worked to is 6 square feet of chimney area for every 30
feet X 8 feet Lancashire boiler up to four boilers ; beyond this 6 square
feet for every additional boiler.

BOILERS. 53

draught; with a draught of -25 inch, from 16 to 20 lbs. of
bituminous coal can be burnt per square foot of grate area per
hour.

The ratio of grate area to heating surface varies, or should
vary, according to the kind of fuel to be consumed and the
draught. The ordinary ratios in Lancashire, water-tube, and
locomotive boilers are approximately as follows : —

Orate Heating
Area. Surface.

Locomotive boiler, , , , • • 1 23-28
Water-tube boiler (Babcock type), , • 1 60-70
Locomotive boiler, , , . , • 1 60-80

The evaporation per square foot of heating surface varies con-
siderably in different types of boiler. Makers of Lancashire and
Cornish boilers usually allow about 1 square foot of heating
surface for every 5 to 10 lbs. of water to be evaporated from
and at 212* F. Makers of the Babcock boiler allow about
1 square foot for every 2 J to 3 J lbs. to be evaporated. In
locomotive boilers with a strong draught 1 square foot of heating
surface will evaporate from 10 to 17 lbs. of water. The actual
evaporation depends on the amount of coal properly burned in
relation to the heating surface, and upon the degree of effective-
ness of the heating surface.

In order that coal may be burnt to the best advantage, the
right amount of air must be supplied, and there must be
sufficient space in which combustion can be carried out. If
insufficient air is supplied, or if combustion is interfered with by
contact with a cool surface, incomplete combusion takes place,
and carbon monoxide or CO is formed. Now, if 1 lb. of carbon
is improperly burned so that it forms CO, it will give only
about 4,330 B.T.U., as against 14,500 B.T.U., which it gives if
properly burnt so as to form carbon dioxide or COg. If, on the
other hand, too much air is admitted to the furnace, the tem-
perature is reduced, and some of the heat given out by the coal
is wasted. Smoke and soot are produced by the incomplete
combustion of the carbon.

The number of heat units which can be transmitted through
a steel boiler plate depends principally upon the difference in
temperature of the gases on one side of the boiler plate and of
the water on the other side, and in a less degree upon the thick-
ness of the plate.

Mr. Blechynden, in a paper read before the Institute of Naval
Architects some years ago, gave the results of a series of ex-
haustive experiments which he made as to the transmission of
heat through plates of various thicknesses and with different

MECHANICAL ENGIKEEBING FOR BEGINNERS.

degrees of temperature on the two sides of the plate,
following are some of the results obtained : —

The

Difference of

Temperature of

the Two Sides

of Plate.

Heat trans-
mitted per !•

difference

per square foot

per hour.

QP.

848
1.013
1,278

626
1,058
1,233

563
1,148

503
723
893

738
1,083

• 12-78
. 15-26
f 20-9

10 89
19-18
21-92

11-90
25-7

11-81 -
16 55
20-65

16-46
25-48

•Plate 1-1875 inch thick.
»» »»
•»> i»

Plate -75 inch thick.
»» >»

Plate -5625 inch thick.
Plate 25 inch thick.

it tt
It tt

Plate -125 inch thick.

In a paper read before the same society a year previously, Mr.
(now Sir John) Durston gave the results of some experiments
undertaken to show the eflfect of grease on the surface of boiler
plates ; the experiments showed that a film of grease caused a
most astonishing decrease in the number of heat units which
could be transmitted through a plate of a given thickness, and
with a given difference of temperature on its two sides.

Feed Water for Boilers and Boiler Compositions. — ^It
is important that the feed water for boilers should be as free
from impurities as possible. Water usually contains a certain
amount of lime, magnesia, and other impurities ; these are
thrown down by the heat in the form of sediment, which be-
comes extremely hard, and has an injurious effect on the boiler.
The incrustation acts as a non-conductor of heat, so that the
water is unable to take away the heat from the steel boiler
plates, and the metal may in consequence get unduly hot ; this
in turn may cause the furnace crown to collapse, or may give
rise to undue expansion, and thus set up dangerous stresses in
the boiler. The non-conducting properties of this scale may also
cause a serious falling off in the evaporative power of the boiler.

BOILERS. 55

It is considered that an incrustation ^ inch thick causes a
falling off of 15 per cent, in the evaporation, and consequently
15 per cent, of the coal is wasted.

As previously mentioned, the impurities in water are thrown
down by heat. This fact is made use of in the Niclausse boiler, in
which the feed water is admitted to the steam space in the form
of spray over a suitable tray provided with a sludge pipe. The
impurities fall into the tray in the form of sludge, and the latter
is, or should be, periodically blown out. The Boby feed-water
heater and detartariser acts on the same principle. Various
mechanical filters are made for removing mud and impurities
merely held in suspension. In these provision is made for
cleaning the filtering material, either by blowing through steam
or by reversing the direction of the current of water.

Various boiler compositions to prevent incrustation are sold ;
the effect of these is to prevent the sediment forming a hard
scale, but these compositions should not be used unless the water
has been analysed, and the user has reasonable grounds for
believing that the composition will do what is claimed for it.
Most of the compositions contain tannic acid. This acid is
effective in cases where the water contains carbonate of lime
and magnesia, but if used in excess is injurious to the boiler
plates. Soda is useful when the water contains sulphate of
lime or acids, but it causes a boiler to prime if used in excess.
The right course, if the water is hard or impure, is to treat it
chemically, or by filtration before it enters the boiler.

Testing Boilers. — Before a boiler leaves the makers' works
it is tested by hydraulic pressure for tightness and strength.
The pressure usually employed is 50 per cent, greater (sometimes
100 per cent, greater) than the working pressure for which the
boiler has been constructed. After a boiler has been supplied
and fixed, the purchaser may desire to test it for its evaporative
performance. This is done by weighing or measuring the water
pumped in, the coal consumed, and the weight of the ashes
removed. Care is taken to see that the level of the water is the
same at the end as at the beginning of the trial, and that the
thickness of the fire is approximately the same at the commence-
ment and end of the trial.

The results of such a trial are usually tabulated in the manner
shown by Table x. Trials are often carried out in a more elabo-
rate manner than is indicated by the table. For instance, the
coal may be analysed to ascertain the number of B.T.U. yielded
by it. The waste gases may also be analysed, and the steam be
tested for dryness. Steam is tested for dryness usually by
means of a throttle calorimeter. The principle of the calori-

MECHANICAL ENGINEERING FOB BEGINNERS.

meter is this : — ^When steam passes through a constricted
opening, it is said to be wire-drawn, and becomes slightly
superheated, the amount of the superheat depending on the
dryness of the steam. Now, the amount by which really dry
steam becomes superheated being known, if the amount of
superheat imparted to the steam under test is ascertained, it
is not difficult to calculate the percentage of moisture in it.

Trial of a Water-tube Boiler for a 260 I.H.P.
Compound Non-oondensing Engine.

Date of test,

Duration of test, ....
Heating surface, ....

Grate area,

Eatio of heating surface to grate area,
Average gauge pressure, .
Temperature of feed water.
Pounds of coal burnt,

„ refuse, ....

„ combustible.
Per cent, of ashes, ....
Coal burnt per square foot of grate, .
Total water evaporated.
Water evaporated per hour.
Water evaporated per pound of coal under

actual conditions,
Water evaporated per pound of coal from

and at 212'* R, ....
Water evaporated per pound of combustible

from and at 212° P., .
Temperature of flue gases.
Draught in inches of water.
Efficiency of boiler, assuming a basis of

14,000 B.T.U. per pound of coal.

22/8/06.
8 hours.

1,506 square feet.
28 square feet.
53-8 to 1.
180 lbs.
158° P.
4,190-4.
297-5.
3,892-9.
7-1.

18-7 lbs.
38,552 lbs.
4,819 lbs.

9-2 lbs.

10-3 lbs.

11-1 lbs.
450\
•5 inch.

71 per cent.

Note. — If an economiser is fixed in connection with the boiler, its
heating surface is given, and the temperature of the water and gases
before entering and after leaving are also given. If a superheater is
used, its heating surface is given, and the temperature of the steam as
it leaves the boiler is also noted.

Results similar to those given in the table would be expected from a
good Lancashire boiler, but the heating surface would be less, and the
temperature of the flue gases would prooably be higher.

CHAPTER IV.

STEAM-BAISING ACCESSOBIES.

We will now consider some accessories which are used in
connection with steam boilers.

Pumps. — In order to force water into a boiler under steam
pressure, a pump or injector is required. When the boiler
is used for driving a slow-speed engine, the latter is usually
provided with its own pump, driven from an eccentric fitted on
the shaft, or from some reciprocating part of the engine, so that
at every stroke of the engine a small quantity of water is forced
into the boiler. A by-pass arrangement is provided, so that any
excess water may be returned to the hot well. When the engine

Fig. 21. — Worthington Duplex feed pump.

is of the high-speed type, or when the installation consists of
several engines or turbines, a separate pump is usually employed.
This pump may be driven by steam or by an electric motor ; by
running it faster or slower a greater or smaller quantity of water
can be fed in to the boilers, and thus the water-level be kept
constant.

When a steam pump is employed, a very common form is that
known as the Duplex direct-acting pump, shown by Fig. 21. In
this pump there are two steam and two water cylinders placed
horizontally side by side ; the steam piston is connected by a rod

MECHANICAL ENGINEERING FOR BEGINNERS.

to the water piston or plunger. The area of the steam piston is
usually twice that of the water piston, consequently the former
is able to drive the latter. The motion of the rod of one steam
cylinder gives the necessary motion to the valve of the other
cylinder. The water enters through the space marked " inlet/'
is forced through the upper valves into the space marked D, and
from thence it descends through a passage placed between the
two pumps to the common discharge, D D.

The Duplex pump is inexpensive and reliable, but is wasteful
of steam, as the steam is admitted right to the end of* every -
stroke, and the steam clearance spaces are large.

A much more economical form of directnacting steam pump is
the Weir (Fig. 21a). In this pump there is usually one steam
cylinder and one water cylinder arranged singly, as shown by
the illustration, or in pairs. The valve is arranged to cut off
steam when the piston has travelled about 75 per cent, of its
stroke, so that during the last 25 per cent, of the stroke the
steam is used expansively. Provision is made for admitting
steam by by-passes during the whole of the stroke when the
pump is first started ; when the pump is well under weigh the
by-pass valves are closed. There are two steam valves, one main
and one auxiliary valve; the latter is actuated by a tappet
motion from the pump rod, and admits steam to either side
of the main valve; the latter is alternately driven backwards
and forwards by the steam, and so controls the action of the
pump.

The following is a table giving some of the standard sizes of
the Weir pumps, having one steam cylinder and one water
barrel : —

TABLE X.

Diameter

Steam

Cylinder.

Diameter

Pump.

Length
Stroke.

Strokes
Mimite.

Size

Suction.

Size

Delivery.

Gallons
Hour.

Inches.

515

810

1,160

1,450

1,730

1,950

3,700

lOJ

4,450

7.540

STEAM-RAISING ACCESSORIES.

If the reader checks the number of gallons delivered according
to the table by the formula given later, he will find that a some-
what small allowance has been made for slip. If, howeVer, the
estimated slip should be exceeded, the number of gallons given

Fig. 2 la. — Weir feed pump.

OF THE

MECHANICAL ENOINEEBINO FOR BEGINNERS.

above can be obtained by running the pumps a little faster, as
the piston speed indicated by the figures is very moderate
indeed. The Weir pump is also made in the compound form
— i,e,j having one H.P. cylinder, one L.P. cylinder, and two
water cylinders.

Another very reliable form of direct-acting pump is the
" Deane," made by the Pulsometer Engineering Company, but
in this pump the steam is not used expansively.

Still another form of steam pump is that of the flywheel
type, as shown by Fig. 22, and made by Messrs. Cameron, of
Leeds, and others. In this pump there are one or more steam

cylinders with corresponding
water cylinders. The valve
controlling the supply of steam
to each cylinder is driven from
an eccentric mounted on the
shaft carrying the flywheel.
With this form of pump the
admission of steam need not
be continued until the end of
the stroke, so that it may be
used expansively; further,
compound cylinders may be
used. The objection to the
flywheel form of pump is that,
should any temporary ob-
struction occur in the delivery
pipes, the energy stored in
the flywheel may cause an
undue rise of pressure in the
pipes.

In electric generating
stations electrically - driven
feed pumps are sometimes
employed ; a small electric
motor running at a high speed
drives the pump through suitable gearing at a moderate speed.
There are, however, certain electrical difiiculties in regulating
the speed of the motor through a sufficiently wide range of
speed, and a by-pass on the delivery side of the pump is a
wasteful arrangement.

The number of gallons of water which should be discharged
by a double-acting pump, if there were no slip, can be found
thus —

Fig. 22. — Cameron pump.

STEAM-RAISING ACCESSORIES. 61

A X L X N

277

= G;

where A = area of pump bucket or plunger in inches.
L =s length of stroke in inches.
N = number of effective strokes per minute.
G =s gallons per minute.

To obtain the number of gallons delivered per hour, the result
must be multiplied by 60.

If the pump is oi the ram type which draws in water once
during every two strokes, the number of effective strokes is
reduced by half. If the pump has more than one barrel, the
result obtained by the above formula must be multiplied by the
number of barrels. If it is desired to find the number of
pounds of water discharged per minute, the number of gallons
must be multiplied by 10.

The formula does not take into account slip — i,e., the per-
centage of water which finds its way back while the valves are
closing, or which leaks past the bucket or ram. It is, therefore,
necessary to make an allowance for this, and in the case of the
average boiler feed pump it is best to assume that only 90 per
cent, of the amount which should theoretically be delivered is
actually pumped. It should be stated that there are 277*27 cubic
inches in a gallon, while in the formula the figure is given as
277. The latter figure is given for convenience of calculation ;
it is sufficiently accurate, considering that a margin must, in any
case, be allowed for slip.

Speeds up to 100 feet per minute are suitable for the buckets
or plungers of boiler feed pumps.

Injectors. — In locomotives the water is usually supplied by
means of an injector. This instrument was invented by a French
engineer named Giffard ; by its use a boiler can feed itself with
water without the intervention of a force pump.

An injector is shown diagrammatically in Fig. 23. The prin-
ciple upon which it works is this : — The steam nozzle is surrounded
by cool water at A ; when the steam passing through the pipe
B meets this water it condenses and forms a partial vacuum ;
this causes the steam in the pipe B to travel at a great velocity,
in order to fill up the vacuum as the steam is at the full boiler
pressure ; the vacuum also causes the water in pipe C to rush
in at a great velocity, the water being driven in by the pressure
of the atmosphere. The velocity acquired by the column of
steam and water is so great that it causes the pressure on the
under side of the check valve to be higher than the pressure

MECHANICAL ENGINEERING FOB BEGINNERS.

Steam

above it, the check valve opens, the condensed steam and water
enter the boiler, and so keep it supplied.

A modem injector (made by Messrs. Holden k Brooke, of
Manchester) is shown by Fig. 24. The injector is of the one^
movement type — that is to say, the admission of steam and
water is simultaneously regulated by the one handle.

On first starting an injector, before the column of condensed

steam and water has acquired
enough energy to open the
check valve, it passes away
through the overflow outlet,
but as soon as the injector gets
to work the overflow ceases.

An injector which is required
to lift its own water, and to
work with fairly high steam
pressures, will not work satis-
factorily if the temperature of
the water at A is higher than
about 80** F. If, however, the
injector is arranged so that the
water flows into it by gravity,
injection water at a higher
temperature may be used,
especially if the working steam
pressure is low. With a boiler
pressure of 50 lbs. per square
inch, the temperature of the
injection water may be as high
afi 140' F. With a boiler
pressure of 100 lbs., water at
a temperature of 120* F. may
be used. With a boiler pressure
of 200 lbs., the temperature of
the injection water should not
be more than 90", even if fed
into the injector by gravity.
Exhaust Steam Iiijectors. — An injector may be worked
by the exhaust steam from a non-condensing engine ; such an
arrangement has the effect of slightly reducing the back pressure
in the engine and of warming the water. If, however, the load
on the engine is very variable the action of the injector is rather
uncertain. The objection to the use of injectors is that there is
a danger of the passages becoming choked with sediment, in
which case the injector naturally ceases to work.

--'Cold ^i,
Wa£'er.—£

Fig. 23.-

-Dlagram showing action
of injector.

STEAM-RAISINO ACCESSORIES.

Feed- Water Heaters and Economisers. — It is uneconomi-
cal to supply a boiler with cold feed water, and any heat which
would otherwise be wasted should be used to heat it. When a

Fig. 24. — Modem injector.

boiler is used^for driving a non-condensing engine, the steam,
after it has done its work in the engine, can be utilised for

MECHANICAL ENGINEERING FOR BEGINNERS.

warming the feed water by means of a simple apparatus called a
feed-water heater. This'ccmsists of a number of tubes placed

inside a shell or drum ; through
these tubes, usually about 1^
inches diameter, the water is
forced on its way to the boiler,
the exhaust steam from the
engine is taken to the shell
before passing away to the
atmosphere; it surrounds the
small tubes through which the
water is passing, and gives up
a large portion of its heat to
the water.

A good feed-water heater is
shown by Fig. 25. It will be
seen that the tubes are free to
expand irrespective of the
outer casing. Any impurities
which may be thrown down
from the water by the heat of
the exhaust steam will fall into
the deposit chamber, from
whence the deposit can easily
be removed. The feed heater
shown is made by Messrs.
Holden <k Brooke.

A feed-water heater of the
kind shown will raise the tem-
perature of the feed from about
60' F. to about 180' F., when
worked at its rated capacity.
If worked below its full capa-
city, the water can be made
still hotter. As will be seen
from the illustration the feed
water passes once up and once
down the tubes ; in the latest
heater made by Messrs. Holden
& Brooke and called their High
Velocity feed heater, the water
is passed six times up and six
times down the tubes; the

CLEANIl

DOoa

Fig. 25. — Feed-water heater.

water is also driven through the tubes at a high velocity, the
result being that the water rolling over on itself, so to speak,

STEAM'RAISING ACOBSSORIES. DO

in its passage through the tubes, extracts more heat from the
steam. With this form of heater the temperature of the feed
is raised from 60' to about 200* when worked at its full rated
capacity, but more energy is expended in driving the water
through. The saving effected through heating the feed water
is approximately 1 per cent, of coal for every 10° by which the
temperature of the feed is raised.

In cases where the exhaust steam is not available for this
purpose, as in a condensing engine, the feed water is often heated
by means of an Eoonomiser. An economiser consists of a large
number of vertical tubes about 4 inches in internal diameter and
9 feet long, placed in a brickwork setting at the back end of a
Cornish, Lancashire, or other boiler; the hot gases are compelled
to pass around these economiser tubes on their way to the
chimney; the feed water is forced through the tubes before
entering the boiler. It has been found necessary to provide
means to keep the tubes free from soot ; to effect this each tube
is provided with a scraper which travels slowly up and down its
exterior; the power to work these scrapers is taken from the
engine by means of shafting, or a small electric motor is provided
for the purpose.

If the gases leave the boiler at a temperature of 650°, an
economiser will extract about 300°, reducing the temperature of
the gases to 350°. The temperature of the feed water passing
through the economiser will be raised by about 150°, say from
62° to 212°, or if the feed water is taken from the hot well, and
its temperature is already 100°, the economiser will raise the
temperature to 250°. ^ ^

Economisers are not used so frequently with water-tube boilers
as with those of the shell type, as the larger amount of heating
surface in a boiler of, say, the Babcock type, extracts a larger
percentage of the heat from the gases; in fact, the portions of the
tubes in a Babcock boiler nearest the chimney may be looked
upon in the light of an economiser, but without the mechanical
scrapers. It is not advisable to reduce the temperature of the
chimney gases below 350°, if a good draught is desired.

Thermal Storage. — ^Another method of heating the feed
water is by means of live steam ; this method is adopted in what
is known as the Thermal Storage System, introduced by Mr.
Druitt Halpin. The thermal storage system is frequently used
in electric generating stations where, during the day time, the
boilers can generate more steam than the engines or turbines
require, but where, during the evening, when the heavy load
comes on, the boilers have difficulty in meeting the demand for
steam. The system briefly is this: — There is a large, drum

66 MECHANICAL ENGINEERING FOB BEGINNERS.

containing a considerable amount of feed water, the drum being
cons ructed to bear the same pressure as the boiler. During the
hours when the engines require only a small quantity of steam, a
portion of the steam generated is passed into the thermal storage
drum, and its heat is given up to the water. The amount of
steam passed is such that by the time the evening comes on with
a heavy demand for steam, the water in the drum will have
received sufficient heat to enable it to turn into steam without
the addition of any further heat, should the pressure fall ; or the
addition of a very small quantity of heat, obtained by passing
the water from the storage drum through the boiler, will cause
the water to evaporate at the full steam pressure.

Superheaters. — The advantage of superheating steam, from
the point of view of economy, has long been known, but it is
only within recent years that superheaters have come into any-
thing like general use. By superheating steam before it leaves
the boiler, loss due to condensation of the steam in the pipes
conveying it to the engine is avoided, and initial condensation in
the engine itself is reduced. As the initial condensation in a
simple steam engine may be as much as 30 or 40 per cent.,
the gain through using superheated steam with such engines
is apparent. In compound or triple-expansion engines, or in
turbines, where the initial condensation is less, the gain
through the use of superheated steam is not so great, but it
is still considerable.

In trials of some high-speed tiiple-expansion engines, which
were carried out a few years ago by the makers, the saving due
to superheating the steam by 100' at the engine stop valve
varied from 14 J to 16J per cent, at full load, and from 20 to
22 J per cent, at one-third load. Speaking generally, it may be
said that with a good compound or triple-expansion engine a
saving of 1 per cent, of steam at full load is effected by every
6 per cent, of superheat given to the steam, while in a steam
turbine a saving of 1 per cent, is effected by every 10' to 12' of
superheat.

The Admiralty recently carried out some trials to ascertain
the value of superheated steam in H.M.S. Britcmnia, and it was
found that at cruising speed, when the engines developed one-
fifth of their power, the saving effected through superheating the
steam by 83' at the engines was 15 per cent.

A superheater consists of a number of small tubes, which are
placed in the path of the furnace gases, and through which the
steam is passed. In Cornish and Lancashire boilers the super-
heater is usually placed in the flue at the end of the boiler;
dampers are provided to prevent the steam becoming too highly

STEAM-RAISING ACCESSORIES. 67

superheated. If the steam is too highly superheated, it car-
bonises the oil used in the engine cylinder and valve chests,
as the best mineral oil will not stand a temperature higher than
650°, and few oils will bear a temperature higher than 600**.

For reciprocating engines the amount of superheat usually
employed is about 100", and occasionally 150" ; thus with steam
at a pressure of 180 lbs. the total temperature is about 480", and
occasionally 530". In a steam turbine the degree of superheat
is limited by the temperature which the bronze blades will safely
bear.

In the Babcock boiler the superheater, as will be seen by Fig.
20, is above the main steam-raising tubes and below the drum ;
in this position it is out of the way of the fierce heat of the
furnace,' and special dampers are not required. The small pipe
leading from the drum to the superheater is for flooding the
latter while steam is being raised.

In the M*Phail & Simpson superheater, which was probably
the first really successful superheater used in this country, the
steam, after being superheated in pipes placed in the flues of the
boiler, was taken through a pipe which passed through the water
space of the boiler ; the effect of this was to take some of the
sting out of the steam, or, in other words, to prevent an excessive
degree of superheat being reached.

In the Cruse superheater the tubes are 6 inches diameter, and
have an internal pipe made of copper through which a stream of
water is passed. This water may be drawn from the water space
of the boiler, from the economiser, or from the cold water feed
mains, and in this way the amount of superheat can be regulated.

Some superheaters are separately fired. The best known
superheater of this type is, perhaps, the Schmidt, in which there
are two sets of coils, an upper and lower. The saturated steam
enters at the top — t.e., farthest from the fire — and travels down
through the coils ; dampers are provided, by means of which the
degree of superheat can be regulated. A separately-fired super-
heater, although conducing to economy, can hardly effect so great
a saving as one making use of gases which otherwise would be
wasted. A Schmidt superheater using waste gases is also made.

Mechanical Stokers. — In cases where several boilers are
installed, or where there is a single boiler the output from which
is fairly constant, it is an advantage to fit each boiler with a
mechanical stoker. This apparatus enables the number of men
stokers in a large power-house station to be reduced ; but what
is of much greater importance is the fact that a good mechanical
stoker allows a very cheap quality of coal to be used and burnt
smokelessly. Small coal known as slack, which, without a

68 MECHANICAL ENGINEERING FOR BEGINNERS.

mechanical stoker is comparatively useless for raising steam, is
now very largely used ; it contains from 10,000 to 12,000 B.T.U.
per lb., and can De obtained at prices varying from three shillings
and sixpence to ten shillings per ton, the price varying with the
locality and with the demand, or otherwise, for this very small
coal. A mechanical stoker adds a small quantity of coal to the
fire at regular intervals, and thus ensures more perfect com-
bustion than is possible when a mass of coal is thrown on at a
time.

There are various kinds of mechanical stokers ; the difference
between each consists chiefly in the manner in which the coal is
put on to the fire. In one, the Bennis, the coal is thrown on in
the form of spray; this is called a sprinkling stoker. In another,
the Vicars, the small coal is pushed forward by means of
plungers ; this form of stoker is called a coking stoker, as the
small coal gets coked at a very early
stage in the process of combustion.

Another form is the underfeed stoker.
In this stoker the centre of the fire
grate is higher than the sides; there
is an opening running down the whole
length of the grate through which coal
is fed up from below by means of a
worm ; the bars fall away on each side
of the opening and air is driven in
between them. Fig. 26 shows a section
Fig. 26.—** Underfeed " through the furnace and bars.

stoker. In this form of stoker the coal gets

coked early and very perfect combustion
is obtained. In most stokers of the above types there is a
device for rocking the fire bars, so that the burning fuel is
gradually carried forward until it reaches the end of the grate,
when it falls over in the form of clinker and ash. In the
Bennis stoker a small blast of steam is employed to force air
into the furnace. Air for the underfeed stoker is driven in
by a fan.

Perhaps the best and simplest form of mechanical stoker is
that of the chain grate type. In this stoker the fire bars are
connected together so as to form an endless chain, which is
moved slowly over drums, thus carrying the fuel forward in a
very regular and even manner. An advantage which this stoker
has over many others is that the grate may be of unlimited
length. Until recently the very smallest form of slack could not
be used with chain grate stokers, but the introduction of a new
form of fire bar has overcome this difficulty.

STEAM-RAISING ACCESSORIES. 69

An objection which is .sometimes urged against a mechanical
stoker is that it is somewhat difficult to force a boiler so fitted,
should an exceptional quantity of steam be required in an
emergency. If, however, the stoker is combined with some
system of blast or forced draught, or if the speed of the chain
grate can be accelerated or retarded, the objection does not hold
good.

Although not necessarily connected with mechanical stokers^
two well-known systems of increasing the draught in a furnace
may be mentioned. They are the Howden system (as used
chiefly in marine work) and the Meldrum blower system. In
the Howden system the air is heated by being passed through
pipes placed in the uptake from the boiler; the heated air is
then supplied under pressure both above and below the firegrate.
In the Meldrum system steam is superheated in a pipe placed in
the furnace, and is then discharged through a trumpet-shaped
blower below the furnace ; by this system very small coal can be
burnt smokelessly and without a mechanical stoker.

Coal Conveying Plant. — The large number of boilers in a
power-generating station, and the use of mechanical stokers,
render it almost essential that the coal should be conveyed to
the hoppers of the stokers by mechanical means. This is usually
effected by means of elevators, conveyers, and occasionally by a
crane and grab. The coal bunkers, as a rule, are placed above
the boilers, and shoots convey the coal from these bunkers to
the hoppers of the stokers.

An elevator consists of a series of pressed steel buckets, about
12 inches wide, which are attached to chains passing oVer drums;
as the buckets travel round the lower drum they come in contact
with the coal, either in a barge, bunker, or other receptacle, and
fill themselves ; when the buckets reach the top of the elevator
they tip over and discharge their contents into a shoot leading
to a conveyer.

A conveyer may be either of the bucket or chain type ; in the
latter the conveyer is merely a steel trough, usually from 9
to 18 inches wide, and from 6 to 9 inches deep. At the
bottom of this trough a flat chain resembling a ladder moves
slowly along; the links of the chain are about 2 or 2 J
inches deep, about | inch thick, and spaced about 12 inches
apart. As the chain moves along it drags the coal with it ; the
trough passes over shoots which lead to hoppers on the boilers,
and in the trough, directly over each shoot, is an opening with a
sliding door ; a certain proportion of the coal, as it travels along
the trough, falls through these openings, the exact amount being
regulated by opening or closing the sliding doors. These

70 MECHANICAL ENGINEERING FOR BEGINNERS.

conveyers will convey coal up an incline of 30" or at an even
greater angle, so that in cases where the coal wharf or siding is
at some distaiice from the boiler-house the coal may be conveyed
up a gradual incline to the bunkers on the first floor, and an
elevator may be dispensed with.

If it is desired to convey the coal at right angles to the main
conveyer, the coal is allowed to drop through an opening in the
trough and fall on to a second conveyer. These conveyers are
driven either by motors, by shafting from the engine-house, or
by a separate engine.

The bucket form of conveyer consists of a series of small
buckets attached to two double flat-link chains ; the chains are
provided with wheels which run on suitable rails. When it is
desired to discharge coal from this form of conveyer, the bucket
is tipped up by means of a cam. Special fillers are provided to
ensure that the coal falls into the buckets only as they pass. A
conveyer of the bucket type requires less power to drive it than
one of the plain chain and trough type.

Oil Filters. — When the steam from an engine is condensed
and is used over and over again for feeding the boiler, it is
necessary to remove the oil from it. Oil filters are of two
different kinds, one designed to extract the oil from the steam
before it is condensed, the other to remove the oil from the
condensed steam or water. Of the former type the best known
examples are the Baker and the Templer-Ranoe. The principles
upon which these work are similar; the steam is expanded into
a large chamber, when some of the heavier loose particles of oil
are thrown down by gravity. The steam then strikes against
baffle plates, and its direction is altered, when more oil is
thrown down. In the Templar-Ranoe separator the baffle plates
are hollow with small lip-like openings, the oil as it trickles
down these baffles enters the openings and is not liable to be
again licked up by the steam. Such mechanical oil filters will
extract about 98 per cent, of the oil from the steam. In the
majority of cases the remaining 2 per cent, is harmless, in others
trouble arises from it. One of the most effective filters for
extracting the oil from steam after condensation consists of
chambers containing sawdust, through which the condensed
steam is passed. The sawdust, however, requires to be removed
and replaced at somewhat frequent intervals. The only way to
remove the whole of the oil from steam is to treat it chemically
after condensation. Plants for this "purpose are, however,
somewhat expensive and occupy a good deal of space.

CHAPTER V.
STEAM PIPES AND VALVES.

Steam Pipes, — The pipes conveying steam from the boilers
to the engines or turbines are usually made of wrought iron or
mild steel, and are lap welded, solidnirawn, or riveted. Some
pipe makers still prefer wrought iron to steel for making lap-
welded pipes, as in their opinion a more reliable weld can be
obtained with the former. Solid drawn steel tubes can be
obtained up to 10 inches diameter, but they are expensive.
Lap welded tubes are made up to 14 inches diameter. Pipes
above 14 inches diameter are usually made of riveted mild steel.
Cast-iron pipes should not be used for steam pressures above
80 lbs. per square inch.

The flanges at the ends of wrought-iron and steel pipes are
made of mild steel, wrought iron, cast iron, or cast steel. They
are usually screwed on to .the pipe, the end of the latter being
expanded and riveted over into a space or recess left in the
flange for the purpose. Wrought-iron flanges are sometimes
welded on, but unless the welding is done thoroughly, a screwed
and riveted flange is to be preferred. The flanges of riveted pipes
of large size are, of course, riveted on. The joint between the
flanges, which are faced, consists usually of a soft copper ring or
of a brass corrugated ring. Any jointing material, such as
asbestos, which may blow out, should be avoided.

Arrangement of Pipes, — In laying out a pipe arrangement,
care should be taken to avoid hollows or pockets where water
can collect. In cases where a pocket is unavoidable, it should
be connected by a drain pipe k) a steam trap. These will be
described later.

Most engineers are fully alive to the danger resulting fronpi
water lying in pipes, should it be necessary for these to dip
down from the boiler and rise again to the engines, and great
care is usually taken to see that such pipes are free from water
before starting the engines, and to keep them well drained while
the engines are running; but there is another arrangement
frequently met with, which is almost as dangerous, and which
is probably responsible for a large proportion of the accidents
occurring through the presence of water in steam pipes. The
arrangement referred to consists of a long horizontal or slightly

72 MECHANICAL ENGINEERING FOR BEGINNERS.

sloping length of pipe with a vertical rise at one end, beyond
which the engines take their steam. The arrangement is shown
by Fig. 27.

The sketch shows a long pipe conveying steam to engines
A and B. Any water which condenses in this pipe is carried
forward by the passage of the steam to the bend C, where it
collects and obstructs the pipe to the extent of perhaps one-half
of its area or even more ; this obstruction may not be serious so
long as the engine supplied from flange A, only, is working, but
should the attendant suddenly start up the engine connected to
flange B, the sudden rush of steam past the bend C will probably
pick up the water lodged there, and carry it forward like a
bullet, possibly wrecking the engine at B or some other engine
further along the line of piping. If such a vertical rise after a

t J t - -^ ^

Fig 27. — Dangerous pipe arrangement.

long length of horizontal piping is necessary, the bend should
have a large pocket with a drain pipe leading from it to a steam
trap.

One occasionally, in past years, received advice to arrange
steam pipes with a gradual slope down towards the boiler, so
that all the water in the pipes might drain back to it. This
advice is radically bad, as a moment's consideration will show,
for with the pipes full of steam under pressure, and the steam
travelling away from the boiler at a rate of perhaps 6,000 feet
per minute, it is practically impossible for any water to travel
back against the current of steam. The plan is bad, for another
reason, viz. : — When the engine is not working, and the boiler
stop valve is closed, any steam condensed in the pipe will flow
back towards the boiler, and will lie either on the stop valve, or
in the pipes adjacent to it. When the boiler stop valve is next
opened, the water collected upon it, unless previously drained
oiF, will have to be got rid of somehow.

The stop valve on the boiler should be arranged as shown by
Fig. 28, and not as shown by Fig. 29.

The right way to arrange steam pipes is to erect them with a
slight fall all the way from the boilers to the engines, and to
provide each engine with a steam separator or dryer. This

STEAM PIPES AND VALVES.

dryer may be merely a receptacle arranged, so as to change the
direction of the flow of steam, and to hold the water as caught.
If the direction in which steam is travelling at a high velocity
is suddenly altered, any large particles of water entrained in it
will be thrown down. The steam dryer should have a gauge
glass to show the amount of water lying in it, and be provided
with a drain cock connected either to a trap, or to a pipe, lead-
ing to a sump or hot well. Any valves on branch pipes leading
out of the main steam pipe should be placed close up to the
latter, so that water may not collect in the branches above the
valves when the latter are closed.

In designing pipe work, another point to be borne in mind is
the expansion and contraction of the pipes due to the difference
of temperature when full of steam, and when empty and cold.
Wrought iron expands '0000067 times its own length for every
degree Fahr. in- ^

crease of tempera- ^ /^|H i

ture between 32° g^ f ^Hll '

and 212*, and as
much as -0000089
times its own
length for every
degree Fahr. when
the range of tem-
perature is between
32° and 500°, so
that if we have a
pipe 50 feet long

Fig. 28. Fig. 29.

Positions of stop valve on boiler.

= 600 inches, and its temperature rises from 50° to 480* (which
temperature is reached with steam at 185 lbs. pressure and 100*
of superheat), we have a difference of 430*. The expansion,
therefore, is 600 x 430 x -0000089 = 2*29 inches, a total ex-
pansion of 2*29 inches in 50 feet of pipes.

It is usual to provide for such expansion by having easy
wrought-iron or mild steel bends at each end of the pipes,
and easy bends where the pipes join and leave the main steam
range. Formerly copper bends were used to take up the ex-
pansion, but as copper rapidly loses its strength under high
temperature, the use of such copper bends has been practically
abandoned.

Where it is impossible to provide bends to take up the
expansion, an expansion joint, consisting of one pipe running
into another of larger diameter, and provided wii^ a gland, is
employed ; the gland is packed with asbestos, or other packing,
in the same way as the gland of a piston-rod is pa^cked.

74 MECHANICAL ENGINEERINO FOR BEGINNERS.

In fixing steam pipes they should be supported on rollers, or
slung in such a way that the contraction and expansion of the
pipes may not strain any of the joints.

When electric-generating stations were first installed in this
country, it was customary for some engineers to employ dupli-
cate steam mains, in order to guard against any interruption to
the supply of current, in the event of the failure of a pipe or
pipe joint. This arrangement necessitated a very large number
of valves, which were in themselves a source of weakness ; and
in any duplicate system there is one point where the two pipes
converge into a i-piece, usually on the separator of the engine.
Should the joint of this Y-piece blow out, the elaborate duplicate
system is rendered useless. It is now more usual • to have one
large pipe into which the boilers feed, and from which the
engines or turbines take their supply. This pipe may be divided
into sections by means of full-way valves, if desired.

Size of Pipes. — ^The size of steam pipes should be such that
the flow of steam in them does not exceed a speed of from 5,000
to 6,000 feet per minute (5,000 feet is preferable for pipes of
4 inches and less in diameter) ; and if the amount of steam to be
carried is known, the area of the pipe can easily be calculated
on this basis by simple arithmetic. The quickest way to make
the calculation is by the formula given below. It should be
remembered that the greater the speed of the steam in the pipes
the greater is the drop in pressure at the far end of the pipe,
or, expressed in another way, a greater amount of pressure is
required to force steam through pipes at a high speed than at a
low one. (The resistance varies with the square of the velocity.)
If the boiler is capable of producing steam at a pressure higher
than is required by the engine, .then there is not the same
objection to employing small pipes ; but if the engine, in order
to develop its full power, requires steam at approximately the
same pressure as that generated in the boiler, then pipes of
ample size should be provided.

The formula for ascertaining the area of a pipe to convey a
certain number of pounds of steam at a given speed is as
follows : —

P X V X 144

S "^'

where P = pounds of steam per minute.

V = volume in cubic feet of 1 lb. of steam at the

pressure to be employed (given in Table ix.)
S = speed permitted in feet per minute.
A = area of pipe in square inches.

STEAM PIPES AND VALVES.

Example. — What size of pipe is required to convey steam to a turbine
using 54,000 lbs per hour, or 900 lbs. per minute, assuming the rate of
flow in the pipe is not to exceed 6,000 feet per minute, the steam pressure
being 150 lbs. ? The calculation is —

900 X 2-71 X 144
6,000

= 5S*5 area.

From a table of areas, it will be seen that a pipe 8| inches diameter has
an area of 58*42, so that a pipe of this size would suffice ; but wrought iron
and steel pipes are not made in odd sizes, so that a*pipe of the nearest even
size —viz. , 9 inches — would be selected.

If the area of a pipe is known, and it is desired to ascertain
how many cubic feet of steam will flow through it per minute
at a given linear speed, the calculation is, of course, as follows : —

-rr-7-- = cubic feet.
144

Example, — How many cubic feet of steam will flow through a pipe
6 inches diameter (the area of which is 28 '27 square inches), if the speed
of the steam is not to exceed 6,000 feet per minute ?

28-27 X 6,000 , ,_, ^. . ^

z-rr-^ — =1,177 cubic feet.

144

If the number of cubic feet of steam is known, it can be
converted into pounds of steam by the aid of Table ix.

The following table, giving the amount of steam in cubic feet
per minute, and in pounds per hour, which will pass through
pipes of various sizes when the speed is limited to 6,000 linear
feet, may be useful. The approximate loss of pressure for every
100 feet of straight pipe is also given : —

TABLE XI.— Rate op Flow, 6,000 Feet per Minute.

steam at 150 lbs. (gauge)

steam at 200 lbs. (gauge)

Diameter

Cubic feet

pressure.

of pipe.

per minute.

Approximate

Lbs. per

drop of

Lbs. per

drop of

hour.

pressure per
100 feet.

hour.

pressure per
100 feet.

Ins.

Lbs. per sq. in.

Lbs. per sq.in.

294

6,514

8,320

5-25

523

11,579

14,802

1,177

26,059

33,311

2-6

2,650

58,671

1-4

75,000

1-75

4,708

104,236

1-0

133,245

1-3

10,600

234,686

•6

300,000

•87

76 MECHANICAL ENGINEERING FOR BEGINNERS.

The number of cubic feet of steam per minute that will flow
through a pipe of a given size and at a given speed in lineai*
feet per minute is, of course, the same, whatever the working
pressure of the steam may be; but as the density of steam is
greater at high than at low pressures, a larger number of lbs. of
high-pressure steam will come through than would be the case
with low-pressure steam ; the pressure or head required to drive
the denser steam through is, however, greater.

Thus, if we take the case of a 6-inch pipe, in which the steam
flows at a rate of 6,000 feet per minute, 26,059 lbs. of steam at
150 lbs. pressure will be delivered with a fall of pressure of
about 2 lbs. in each 100 feet of straight length of pipe ; but with
steam at 200 lbs. pressure, 33,311 lbs. will come through, but
the fall of pressure will be about 2*6 lbs. per 100 feet, the
number of cubic feet coming through being 1,777 in each case.

The generally-accepted formula dealing with the flow of steam
in pipes is given below,* but this formula is interesting as
showing what the steam should do in smooth pipes, rather than
what it actually does in practice.

In actual working practice there are many factors which may
entirely vitiate the results of calculations based on elaborate
formulae which cannot well take these factors into account. For
instance, in a long range of straight steam pipe there is, at a
distance of every 12 feet or so, a tiny groove formed by the
space required for the packing placed between the flanges. This
packing ring cannot be made of the same internal diameter as
the pipe, on account of the danger of a part of the packing
projecting into the bore of the pipe and thus causing an obstruc-
tion. These tiny grooves cause eddies in the steam and tend
to impede its flow. Again, in erecting the pipes one cannot be
sure that the centre of each length of pipe will coincide
absolutely with that of its neighbour. There is a certain amount
of clearance in the flange bolt holes, and it is hardly probable
that all the pipes in a long range are absolutely true one with
another. The wetness or dryness, too, of the steam affects its
rate of flow.

The obstruction to the flow of steam caused by a glpbe valve

* W = 87 /^y D iPi - rij) d^
3-6\

W = weight in lbs. per minute.

D = weight per cubic foot of steam at the working pressure.
Pi and P2 = the initial and final pressures.
L = length of pipe in feet.
d = diameter of pipe in inches.

STEAM PIPES AND VALVES. 77

is sometimes stated to be equivalent to so many feet of straight
pipe, varying from 50 feet in a 3-inch pipe to 80 feet in a 12-inch
pipe, but if the globe valve is not drained, and water accumulates
in it as shown by Fig. 30, no formula will indicate what
obstruction is caused.

Even if properly drained, the roughness and shape of the valve
body will afifect the rate of flow considerably.

A method sometimes adopted by a draughtsman to settle the
size of steam pipes, is to ascertain the diameter of the inlets
upon all the engines to be supplied with steam, and which will
be working at the same time, and then to arrange for his steam
pipe to have a corresponding area. For instance, should there

Fig. 30.— Stop valve passage obstructed by water.

be two engines each with an inlet of 6 inches diameter, and one
with an inlet of 5 inches diameter, the total area of these inlets
amounts to 76*1 square inches; the draughtsman turns to a table
of areas and finds that a pipe 10 inches diameter has an
area of 78*5 square inches; he then settles on this size of
pipe. If there are a large number of engines this method
gives a size of pipe larger than is really needed, as the engine
maker, in settling the size of his inlet, usually allows a little
margin.

The principal objection to the use of pipes of unduly large
size, is the heavy first cost, both of pipes and valves. There

A good empirical formula for giving the flow of steam in pipes is as

follows : —

a = an ^y x F x 144 x~D .

where S = speed (in linear feet) of the steam per second.

V - volume in cubic feet of 1 lb. of steam at the working pressure*

F = fall in pressure.

D = diameter of the pipe in feet.

L = length of the pipe in feet.

MBCHANICAL ENGINEERING FOR BEGINNERS.

is also a greater loss from radiation with a large pipe than
with a small one. It may be mentioned that the loss from
this source is about 13 or 14 B.T.TJ. per square foot of an
uncovered pipe. This loss may be reduced to one varying from
lA to 4 B.T.U. per square foot if the pipe is covered with one
01 the various non-K^onducting compositions sold for the purpose.

On the other hand, a large steam pipe provides a certain
reservoir of steam ; this is an advantage when the steam is
required for a slow-speed reciprocating engine, which demands a
large supply of steam while the admission port is open, and
requirea no steam while the port is closed. This matter will be
referred to again under valves.

The effect of superheating steam is to increase its volume, not
its pressure, but superheated steam travels along a pipe with less
friction than wet steam and prevents any obstruction due to
water, such as is shown by Fig. 30.

The increase of volume caused by superheating steam is
approximately as follows : —

100» Superheat.
Increaae of volume.

150" Superheat.
Increase of volume.

100 lbs. pressure, .

160 „ „ . , .

200 „ „ . . .

12 -6 per cent.
120 „
11-8 „

19 per cent.
18 „
17 „

If, therefore, pipes are designed so that a certain weight of
saturated steam, at 200 lbs. pressure, should flow at 6,000 feet
per minute, then the same weight of steam superheated by 100°
will require to flow at about 6,708 feet per minute. There
would be no real objection to this increased rate of flow, as
superheated steam approaches more nearly to a perfect gas than
saturated steam, and its friction is less.

Strength of Pipes. — The tensile stress in the material of a
pipe or cylinder under internal pressure is found by the follow-
ing formula : — -

D X P

S =

2 X T

where S = Tensile stress per square inch.
D = Diameter of pipe in inches.
P = Pressure in pounds per square inch.
T = Thickness of pipe in inches, or parts of an inch.

In the case of lapwelded wrought-iron and steel pipes, it is
not customary to allow a stress equal to the safe working stress

STEAM PIPES AND VALVES.

of the metal, as the strength of the weld and the stiffness of the
pipe generally have to be taken into account.

The usual thickness of wrought-iron and steel pipes suitable
for working up to pressures of 200 lbs. per square inch is as
follows: —

TABLE XII.

Lapwelded.

Diameter
of Pipe.

Solid Drawn
Steel.

Wrought Iron.

steel.

Inches.

About ^ less

thickness

than

lapwelded

steel

tubes.

If the reader will apply the formula given above to the pipes
in the table, he will find that with a working pressure of 200 lbs.
the stress in the metal is only about J ton per square inch in the
case of the 3-inch pipe; below 1 ton in the case of the 4-inch,
5-inch, and 6-inch pipes ; and less than l^ tons in the case of the
12-inch pipe.

As in practice the metal is stressed to such a small extent
there is but little advantage in employing mild steel for lap-
welded pipes if a better weld can be obtain^, which is doubtful,
with wrought iron.

Lapwelded pipes of less than 3 inches diameter, whether for
steam or gas, are made of a uniform outside diameter — this
diameter will be found in the table of gas threads (Table v..
Chap. II.)— and the inside diameter is made greater or smaller
according to the pressure the pipe is required to withstand.
Thus a 2-inch steam pipe is of a slightly smaller internal
diameter than a 2-inch gas pipe, the outside diameter being
the same. This rule does not apply to hydraulic tubes, which
are of considerable thickness.

Size of Flanges. — Until quite recently, every engineer has
used his own standard dimensions for pipe flanges. This has
caused a great deal of inconvenience, especially to valve makers.

MECHANICAL ENGINEEBING FOR BEGINNERS.

Recently, however, the Engineeering Standards Committee, a
body composed of members of the Civil and Mechanical
Engineers' Institutions, of the Institution of Naval Architects,
and of the Iron and Steel Institute, drew up a table settling
the diameter and thickness of flanges, the number of bolts to be
used, and the pitch circle; both for steam pipes suitable for
pressures up to 225 lbs. per square inch, and for exhaust pipes,
or pipes suitable for working up to pressures not exceeding 55
lbs. per square inch.

These tables are appended and may prove useful; many
engineering firms have already adopted them, and others intend
to do so as soon as existing stock has been disposed of.

TABLE XIII. — Dimensions op British Standard

Pipe Flanges.

For steam pressure up to 225 lbs. per square inch.

Internal

diameter of

pipe.

Diameter of

Number

Diameter

Thickness of Flange.

Flange.

Bolt arele.

of Bolts.

Cast iron

or Wrought

Iron.

Steel or
Bronze.

Inches.

Inches

Inches.

104

13J

114

144

12i

1; 1

1" ■

1; ;

19i

171

21i

194

2li

264

334

30i

STEAM PIPES AND VALVES.

TABLE XIV. — Dimensions of British Standard
Pipe Flanges.

For steam pressures up to 55 lbs, per sqtiare inch.

Tnf ^rnol

Thickness of Flanges.

Xlll/ClllIU

Diftnidter of

Diameter of

Number

Diameter

diameter of
pipe.

Flange.

Bolt Circle.

of Bolts.

Cast Iron.

steel or
Bronze.

Inches.

lOi

144

i2i

201

184

22|

20i

25j

27}

25}

324

29|

Water Hammer. — ^When steam at a high pressure ig
admitted to a pipe in which a certain amount of cold water
is lying, a succession of sharp reports is heard, just as though
blows were being struck on the inside of the pipe by a hammer ;
sometimes the blows are sufficiently strong to fracture the pipe
or a neighbouring valve.

The precise action of water hammer has recently been
investigated by Mr. Strohmeyer, who used glass tubes for his
investigations. Mr. Strohmeyer found that when steam was
admitted to a pipe containing water, waves were set up in the
latter which imprisoned and isolated portions of the steam.
The isolated portion of the steam being in contact with cold
water, and no further supply of heat being able to come to its
rescue, condensed, the water then rushed into the space formerly
occupied by the steam and caused a sharp blow.

In addition to this action of water hammer proper, the
presence of water in a steam pipe, as explained earlier in this
chapter and illustrated by Fig. 27, may lead to disaster. In

MECHANICAL ENGINEERING FOR BEGINNERS.

such a case the water is picked up by the rush of steam and acts
as a projectile.

Sometimes an accident is caused through the attendant not
realising what goes on in a steam pipe after the engine and
boiler valves have been closed. What happens is this : — ^After
the valves have been closed for some time, and heat has been
lost by radiation, the steam condenses and forms water ; if the
valves are tight no air can get into the pipes and a vacuum is
formed. Now, if the drain cocks on the pipe are opened before
a small quantity of steam is admitted, air will pass into the
pipes, but water will not come out against atmospheric pressure.
The attendant, seeing no water coming from the pipe, may
conclude that it is free from water, and may turn on the steam
too suddenly, with disastrous results. At an enquiry recently
held by the Board of Trade upon an accident due to water
hammer, the attendant said that when he opened the cocks he
saw no water coming from the pipes, but heard a hissing noise ;
even this did not convey to him the fact that there was a
vacuum in the pipes, and that the hissing sound he heard was
due to air rushing in.

Exhaust Pipes are usually made of cast iron, as the pressure
they have to withstand is low ; when the engine is exhausting
to the atmosphere the pressure in the pipes is not much above
that of the atmosphere. When the engine is condensing the
pipe is subject to a crushing stress not exceeding 15 lbs. per
square inch.

In calculating the strength of a wrought-iron pipe, we said
that the weld and general stiffness of the pipe had to be taken
into consideration. In a cast-iron pipe there is of course no
weld, but in casting the pipe the core may have shifted, and one
side of the pipe may be much thinner than the other. Even if
the pipe were of the same thickness throughout, a pipe calculated
on the basis of the safe stress for east iron would be much too
thin, especially in cases where the pressure is low. In practice
the thickness of exhaust pipes is approximately as follows : —

Diameter.

Thickness.

Diameter.

Thickness.

Inches.

Inch.

Inches.

Inch.

STEAM PIPES AND VALVES.

Steam Traps are devices for allowing water automatically to
leave a pipe containing steam under pressure without permitting
steam to pass. Steam traps are of two types — viz., the bucket
and expansion types. The former depends for its action upon

Fig. 31.— Bucket trap.

a floating bucket; the latter depends upon the diflPerence of
temperature between the water which has condensed and that
of the live steam. Fig. 31 shows a bucket steam trap as made
by Messrs. Holden & Brooke, of Manchester. At starting, the
trap is partly filled with water, which causes the floating bucket

Fig. 32. — Expansion trap.

to rise and close the outlet valve. When water comes down
from the steam pipe it gradually fills the shell, reaches the top
of the bucket, and flows into it; when the bucket is full its
buoyancy has, of course, disappeared, and the weight of the

84 MECHANIQAL ENGINEERING FOR BEGINNERS.

bucket is sufficient to open the outlet valve. The pressure of
steam on the surface of the water then drives the latter out
of the bucket, up the central outlet pipe; when it has been
expelled the weight of water surrounding the bucket causes the
latter to rise and to again close the outlet valve.

Fig. 32 shows a Brooke expansion trap. In this trap steam
can blow freely through the central tube until the steam has
warmed it, when the expansion of the tube causes the outlet
valve to close. When water comes down from the steam pipe it
cools the tube and causes it to contract, consequently the valve
opens and the water is blown out. A diflFerence of 5** F. in
temperature is sufficient to make the trap act ; in fact, it is so
sensitive that if a little water is sprinkled off the hand on to the
central tube when full of steam, the valve opens, spits out steam,
and then closes.

The objection to the expansion form of trap is that the valve
and seat after a time get cut by the outgoing steam and water,
and the valve leaks. In the Brooke bucket trap the outlet
valve is always under water, and is given a rotary motion, so
that it grinds itself in at every discharge. The bucket form of
trap is considered the more reliable of the two kinds, but it
takes up more space, and is more expensive than the expansion
form of trap.

Steam Stop Valves. — Fig. 33 shows a screw-down, or globe
right-angled boiler stop valve, having a renewable seating ; that
is to say, the valve seat is not made solid with the valve body,
but a separate metal seat is forced in to the body, and is held
in by set screws. The reason for making the seat renewable
is that a valve seat, especially if the valve is used for regulating
the flow of steam, gets scored or cut by the action of the steam,
and the valve is no longer tight when closed.

In the stop valve illustrated, the valve and seat can be renewed
when scored without necessitating the renewal of the body.
The valves and seats are usually made of some alloy which does
not corrode ; gunmetal was formerly used, but this alloy is not
suitable for superheated steam. One firm of valve makers
(Messrs. Templer & Ranoe) use a special nickel alloy for their
valves and seats, while another firm (Messrs. Hopkinson) use an
alloy which they call platnam ; this must not be confused with
platinum. Platnam is doubtless a fancy name. The bodies of
valves for high-pressure steam should be made of cast steel and
not of cast iron.

Valves of the screw-down pattern should be arranged so that
the steam assists the valve to open, and constructed so that the
screw forces the valve on to its seat ; if a valve is constructed so

STEAM PIPES AND VALVES.

that the spindle draws the valve on to its seat, it is difficult ,tO
keep the valve tight.

Fig. 33. — Screw-down stop valve.

Fig. 34 shows a very good form of valve of the gate, or
straight-through type. The valve shown is known as the Stirling

MECHANICAL ENGINEERING FOR BEGINNERS.

(made by Templer k Ranoe). It will be seen from the illustra-
tion that when the valve is closed the wedges force the two

Fig. 34. — Straight-through gate stop valve.

valve faces outwards against the seatings. The two wedges
ensure the upper and lower portions of the valve being for^
equally against the seats. The Stirling is a double-faced valve —

STEAM PIPES AND VALVES. 87

i,e., both valve faces make a steam-tight joint. In valves of
somewhat similar type, suitable for water and for exhaust steam,
and known as sluice valves, one face only of the valve frequently
makes the joint, but double-faced sluice valves are also made.

It is usual to provide large valves of the gate type with a
small by-pass valve, so that the pressure on both sides of the
large valve may be equalised before opening it. This by-pass
valve is useful for admitting a small quantity of steam to the
pipes to warm them up, before opening the main valves.

Valves of the screw-down globe pattern are generally used
upon boilers or engines where it may be necessary to use the
valve for regulating the flow of steam. Valves of the gate type
are generally used in the range of pipe te shut one portion ofl^.
A valve of the gate or straight-through type causes less obstruc-
tion to the passage of the steam than one of the globe type.

Various forms of valve have been designed to get over tlie
cutting action of the steam ; they usually take the form of two
valves in one body, one valve opening before steam can pass the
second ; the first valve therefore does not get cut by the action
of the steam. It should be explained that the cutting action
does not take place when the valve is fully open, but chiefly
when opening, or when the valve is kept partially closed for
regulating purposes. The "T. R." valve (made by Messrs.
Templer & Kanoe) was probably the first vah-e of this kind ; it
is rather expensive, but remains tight for a very long time
Messrs. Hopkinson also make a valve of a somewhat similar
kind which is called the " centre pressure " valve.

Hopkinson-Ferranti Valve. — A steam valve, which has
recently been introduced and largely advertised, is the Hopkin-
son-Ferranti valve. The bore of this valve is gradually reduced
by a nozzle-shaped body, until the bore is only half the diameter
of the pipe to which the valve is attached. The valve itself is,
of course, at the smallest portion of the bore ; beyond the valve
proper the body gradually opens out in a suitably proportioned
manner until the full area of the pipe is again reached. The
principle upon which the valve is constructed, as given by the
makers, is as follows: — "Converting the pressure of the fluid
into velocity and reconverting the velocity into pressure, thereby
passing an amount of steam equal to the full carrying capacity
of the pipe." The makers claim that the val ve is lighter and
cheaper than one having the full opening of the pipe, that there
is less risk of leakage, and that the valve causes practically no
drop in the pressure of the steam passing through it. The
author learns, from independent sources, that when a single
valve of the kind is employed the drop of pressure is almost

88 MECHANICAL ENOINEERINO FOR BEGINNERS.

negligible, provided the valve is fully opened; but if the
attendant should fail to open the valve to the very fullest,
extent, so as to cause the slightest obstruction to the steam
where it flows through the throat of the valve at a high velocity,
then the drop oi pressure is serious.

It can haridly be seriously contended that there is no loss in
the conversion of pressure into velocity and reconverting velocity
into pressure. It is difficult to imagine steam flowing through
a long corrugated pipe, however accurately the corrugations may
be propoi*tioned, as freely as through a plain pipe. Joule's law,
given in the chapter on steam turbines, reads — "When a gas
expands without doing external work, and without taking in or
giving out heat, its temperature does not change." We know
that when steam is passed through a constricted opening it
becomes slightly superheated and expands, and it is difficult to
see how the makers can prevent the steam from giving up some
of this superheat to the valve, and so losing it by conduction
and radiation.

Apart from theoretical considerations there is one point
which should not be overlooked by those using this form of
valve. Every length of horizontal pipe placed between two
such valves should be properly drained, otherwise a pocket is
formed in which water can lie.

Isolating Valves. — Another form of valve used on boilers in
power-generating stations is the isolating valve. This valve will
allow steam to leave a boiler, but will not allow any to re-enter.
Thus if several boilers deliver steam into one common steam
main, and one boiler should develop a serious leak, or burst a
tube, that boiler only will be put out of action, if fitted with an
isolating^ valve, as this valve will prevent steam entering from
the other boilers. Isolating valves sometimes give trouble by
hammering and breaking. This trouble is often experienced
when the boilers supply steam to slow-speed reciprocating
engines, and when the pipes are of small size. The explanation
is doubtless as follows : — When the steam port of the big slow-
speed engine opens steam travels along the pipe at a very high
velocity, the cut off then suddenly takes place, and the flow of
steam at one end of the pipe is checked ; the steam, which was
in motion in the pipes, banks itself up, so to speak, and the
pressure rises higher than that in the boiler. Steam then flows
back to the boiler, and closes the isolating valve with a sharp
blow. Such a blow repeated sixty or seventy times a minute
naturally causes the valve to collapse in a short time.

CHAPTER VI.
THE STEAM ENGINE.

(Part I.)

It is beyond the scope of this book to describe in detail a great
variety of steam engines ; all that is attempted is to make clear
the principles upon which such engines work, and to show how
the ordinary calculations connected with them are made.

Fig. 35 shows a vertical double-acting engine with cylinder
and valve chest in section. The valve shown is of the plain D
type, as it is necessary to understand the action of this form of
valve before considering valves of the piston or other types.
The action of the engine is this : — Steam from the boiler enters
through an inlet at the back of the valve chest A; the slide-
valve S, in the illustration, is just beginning to uncover the
steam port leading from the valve chest to the upper side of the
piston B ; the valve will continue to move downwards until the
port is fully open, when it will begin to move in an upward
direction. The pressure of steam on the piston B causes it to
descend and turn the crank C and crank shaft D towards the
spectator by means of the piston-rod E and connecting-rod F.

On the crank shaft is fitted an eccentric H, which gives motion
to the valve by means of the eccentric-rod J, and valve-rod K.
The eccentric is set in such a position, and the valve is so
proportioned, that when the piston has reached about three-
quarters of its downward stroke the slide valve will have
travelled upwards sufficiently far to prevent the admission of
any more steam. This is called the point of cut-off; after the
cut-off has taken place, the steam which is above the piston
expands and forces the piston down to the end of its stroke. A
flywheel, L, is mounted on the crank shaft, the stored energy of
which carries the crank over the dead centres (i.e., the position
in which the piston is at the extreme end of the stroke), by
which time the slide valve will be in such a position that it is
just opening the port leading to the underside of the piston ; the
slide valve will continue to open the port and the piston will
rise ; at three quarters of its upward stroke steam will be cut
off, and the expansion of the steam beneath it will cause the
piston to complete its stroke.

MECHANICAL ENGINEERING FOR BEGINNERS.

It will be seen from the illustration that when one port is
uncovered to admit steam, the slide valve places the other port

Fig. 35. — Vertical single-cylinder engine.

in communication with the central port or passage M, which, in
a single-cylinder engine, leads to the atmosphere or condenser,
and in a compound engine to the second cylinder.

THE STEAM ENGINE. 91

The port through which the steam exhausts is closed by the
valve slightly before the piston reaches the end of its stroke, so
that the imprisoned steam acts as a cushion, and helps to bring
the piston to a standstill, and to restart it on its reverse stroke.
In a condensing engine this cushioning effect is lost to the
cylinder placed in connection with the condenser ; this accounts
for the fact that certain high-speed engines are much more noisy
when condensing than when exhausting to the atmosphere.

It may be noticed that the faces of the slide valve are wider
than the steam ports; the difference is called the "lap"; the
amount by which the valve faces project beyond the ports in an
outward direction when the valve is central, is called " outside
lap." The amount of outside lap and the position of the
eccentric regulate the point of cut-off. If, when the slide valve
is in a central position, the valve faces overlap the ports on the
inside, it is called " inside lap."

It will be noticed that, although the piston has not commenced
its downward stroke, the valve has slightly uncovered the steam
port ; this is called "the lead of the valve." Thus, if the port is
opened by one-eighth of an inch when the piston is still at the
highest part of its stroke, the valve is said to have one-eighth
inch lead.

By cutting off the admission of steam fairly early, and allowing
it to work expansively, considerable economy is effected, but
with a single D slide valve, if sufficient lap is given to make the
cut-off take place before about two-thirds of the stroke, the
exhaust is closed too early, and too much compression results.

With the D-form of slide valve the pressure of steam on the
back of the valve is more than sufficient to hold it up against
the valve chest face. Thus a small valve, say 5 inches by 4
inches, has an area of 20 square inches, and if the steam pressure
in the valve chest is 120 lbs., a pressure of 2,400 lbs., or over a
ton, will be exerted on the back of the valve. This great pres-
sure renders the plain slide valve unsuitable for cases where the
steam pressure is high, or where the engine runs at a high speed.

In an engine designed for even moderately high speeds or
high steam pressures a valve of the piston, or other type to be
described later, would be used. A valve of the piston type is
shown by Fig. 36. With this form of valve a liner is employed,
in which the ports can be cut more accurately than is possible
in the valve-chest casting; bars are left, so that, if rings are
used in the piston valve, they will not enter the ports.

These bars obstruct, to a certain extent, the area of the port,
and to get the required area a somewhat large piston valve is
necessary. The piston valve is usually made about half the

MECHANICAL EKGIKEERIKO FOB BEGINNERS.

diameter of the cylinder, and such a large valve is inadmissible
in the case of the low-pressure cylinder of a big compound or
triple-expansion engine.

When a plain slide valve is used for the low-pressure cylinder

Fig. 36.— Cylinder with piston valve.

Fig. 37. — Reversing gear. Fig. 37a. — Setting eccentrics.

THE STEAM ENGINE.

of a large engine, it is usually balanced— t.e., the valve and valve
chest are constructed so that the steam pressure acts upon a
small area only of the back of the valve. Fig. 38 shows the low-
pressure cylinder of a large vertical engine, for marine or land
use, fitted with a double-ported balanced slide valve. In the
example shown, steam is prevented from pressing upon a large

Fig. 38. — Low-pressure cylinder, with balanced slide valve,
assistant cylinder, and metallic packing.

portion of the valve by means of the ring R. This ring fits into
a circular ring at the tack of the valve, and is kept steam tight
by means of piston rings. The face of the ring next the valve
chest cover and the inside of the latter are planed, the ring is
kept tightly up against the valve chest cover by the steam
pressure acting at K. Should any steam leak past the ring R,
it is taken away to the condenser through the opening C.

There are many other methods of preventing the steam from

94 MECHANICAL ENGINEERING FOB BEGINNERS.

pressing upon the back of the valve. In many cases the back
of the valve is planed, and the ring B rubs on it. Instead of
using piston rings, the ring R is frequently packed by means of
a gland, as shown by the inset at the top of the illustration. In
the earlier methods of balancing a gland was not employed, the
ring merely fitted into a recess in the valve chest cover, the
recess being filled with asbestos packing. This was unsatisfac-
tory, for if the fitter put too much packing into the recess, the
ring was forced down too hard on to the valve; if too little
packing was used, steam leaked by.

In modern marine practice the valve, in addition to being
balanced in the manner described, is usually provided with an
assistant cylinder placed at the top of the valve chest, as shown.
The object of this assistant cylinder is not only to carry the
weight of the valve, thus relieving the eccentric straps, reversing
link, pins, &c., from the weight, but also to assist the valve to
rise, and to force it gently down at the right moments. In the
" Joy" assistant cylinder the admission of steam is effected by a
reduction in the size of the valve-rod, so that at one portion of
the stroke steam is admitted to the underside of the piston;
when the piston has reached the top of its stroke steam is
admitted to its upper side by means of slots, and the piston is
forced gently down.

In another form of assistant cylinder — viz., the " Lovekin" —
a separate valve is used to distribute the steam to the cylinder,
and steam is taken from the intermediate receiver so as to
obtain it at higher pressure, thus enabling a smaller cylinder
to be used.

The assistant cylinder is a step in advance of the plain
cylinder, which was formerly fitted to the low-pressure valve
chests of marine engines. This plain cylinder was open at
one end to the valve chest, so that steam pressed constantly
on the underside of the piston which carried the main slide
valve.

The spaces S S in the slide valve itself are open to the steam
in the valve chest, so that the valve is double ported. This
construction reduces the amount of valve travel for a given size
of port opening. In the illustration the valve is fully open,
admitting steam to the upper side of the piston, and placing the
lower side in communication with the condenser through the
exhaust passage E. A facing piece of hard cast iron, in which
the ports are accurately cut, is placed between the valve and the
main casting.

The illustration shows the piston- and valve-rods packed by
means of metallic packing, which will be described later. The

THE STEAM ENGINE. 95

^linder cover is fitted with a spring relief valve shown at A.
The object of this relief valve is to allow any water to escape
should such have accumulated on the piston. The piston is
provided with a separate ring in which the Ramsbottom piston
rings are placed. This admits of the rings being renewed with-
out the necessity of removing the piston from its rod.

Beyersing. — When the eccentric is fixed to the shaft in one
position, it will allow the engine to rotate in one direction only.
If the engine is required to rotate in the opposite direction as
well, it is necessary to have two eccentrics, one keyed to the
shaft in a position to make the engine run in one direction, the
second in a position to make it run in the opposite direction.
The rods leading from the two eccentrics are connected to
opposite ends of a link. The valve spindle or rod is connected
to a block placed in the slotted link, the link itself can be
shifted so that the valve-rod may be placed in connection with
either of the two eccentrics.

Fig. 37 shows the ordinary Stephenson link motion. One
eccentric is in its uppermost position, and owing to the position
of the link it is this eccentric which is actuating the valve. It
will be seen that the lower port of the steam chest (Fig. 36) is
uncovered to the steam, while the upper port is in com-
munication with the exhaust. If now the link is pushed over
to the left in the direction of the arrow, the valve will be drawn
down and the upper port will be uncovered to the steam, while
the lower port will be placed in communication with the
exhaust. The ^haft in Fig. 37 is at right angles to the position
it would occupy if it were working in connection with the
cylinder shown oy Fig. 36 ; the two views have been so placed
to show clearly the action of the two eccentrics and link, and
their relation to the valve.

In actual practice the eccentrics are not set at the highest and
lowest positions as shown. They are set at an angle of 90°, plus
the angular advance, in advance of, or behind the crank. The
actual setting of the eccentrics is shown by Fig. 37a, where the
diameter of the circle represents the travel of the valve.
A = lap, B = lead. The angle K is called the "angular
advance." A drawing similar to that shown by Fig. 37a is sent
from the drawing office to the shops with the dimension A + B
given, and the diameter of the circle. This enables the fitters
to set the eccentrics in the correct positions.
, An engine such as that shown by Fig. 35 would not work
very economically as regards steam consumption, and would
only be used in cases where simplicity and small first cost
happened to be of more importance than economy of steam and

96 MECHANICAL ENGINEERING FOB BEGINNERS.

fuel. Such an engine might be used for driving a fan at a
colliery where fuel is cheap, or it might be Used in cases where
the work is intermittent, and where the somewhat excessive
consumption of fuel during the short periods when the engine
is at work is of relative unimportance.

A single-cylinder engine working with a steam pressure of
75 lbs. and exhausting to the atmosphere, would use between
30 and 35 lbs. of steam for every indicated horse-power per
hour ; so that if the engine indicated 50 horse-power, the boiler
supplying the steam would have to evaporate between 1,500 and
1,750 lbs. of water per hour.

By indicated Juyrse-poiver, or I.H.P., is meant the power
actually exerted by the steam in the cylinder without taking
into account the engine friction.

By brake horse-poioer (B.H.P.), or effective horae-power, is
meant the actual horse-power delivered by the crank shaft of the
engine. Thus an engine giving 100 indicated horse-power will
give only about 90 to 95 brake horse-power, the remainder being
lost in friction. If the engine gives 90 brake horse-power for
every 100 indicated horse-power, its mechanical efficiency is said
to be 90 per cent. If it gives 95 brake horse-power for every
100 indicated horse-power its mechanical efficiency is 95
per cent.

This mechanical efficiency has nothing to do with the
consumption of steam or thermal efficiency. A single-cylinder
engine which is wasteful in steam has often a higher mechanical
efficiency than a three-cylinder, triple-expansien, condensing
engine which is very economical of steam. In comparing the
merits of two engines, it is necessary to look at the con-
sumption of steam per brake horse -power, or, if the
consumption is given per indicated horse-power, it is necessary
to ascertain the efficiency of such engine, and to convert the
figures into consumption of steam per brake horse-power. It
is important to remember that because an engine is claimed
to have a very high mechanical efficiency, it does not follow that
it is an economical engine to use. Before proceeding with the
study of compound and triple-expansion engines, and going into
the questions of economy of steam, &c., it may be well to state
how the horse-power of a single-cylinder engine may be worked
out.

The indicated horse-power of a double-acting engine is found
by multiplying twice the stroke (in feet) by the number of
revolutions per minute, by the area of the piston in inches, and
by the mean pressure exerted on the piston, and dividing the
result by 33,000. Stated as a formula it is expressed thus —

THE STEAM ENGINE. 97

jgp _ 2xSxRxAxP
' ' * 33,000 '

where S = stroke in feet.

R = revolutions per minute.
A = area of piston in inches.
P = mean pressure in lbs. exerted on the piston.

Let us take an actual case, say of an engine as shown by Fig.
35, the cylinder of which is 10 inches in diameter, the stroke 8
inches, the number of revolutions 300 per minute, the initial
steam pressure as it enters the cylinder 75 lbs., the cut-off takes
place at about "65 of the stroke, and the average mean pressure
on the piston during the whole of the stroke is 50 lbs.

As the stroke has to be worked out in feet or parts of a foot
before we can make the calculation, we must find out what part
of a foot the stroke of the engine — viz., 8 inches — is. If we turn
to the decimal equivalents given at the end of the book, we see
that 8 inches is -666 of a foot. We must also find out what the
area of a 10-inch piston is ; this we see from the table of areas is
78*5 square inches.

The calculation now is

2 X -666 X 300 X 78-5 x 50 ^ .„,.
33,000

The answer is 47*5 I.H.P. If the mechanical efficiency of the
engine is 90 per cent., then the brake horse-power will be
47-5 X 90 -^ 100 = 42-75 B.H.P.

To obtain an initial pressure of 75 lbs. in the cylinder the
boiler pressure would probably require to be 90 lbs. per square
inch, as there is usually a drop of from 5 to 10 lbs. between the
boiler and engine stop valve, the drop depending upon the length
and diameter of the steam pipe, as explained in the previous
chapter. There should also be a difference of 5 lbs. on the two
sides of the engine governor, if good governing is desired.

Having got 75 lbs. initial pressure we cannot, however, count
on this pressure all through the stroke, as after the point of
cut-off, which in this case we have assumed to take place at '65
of the stroke, the steam expands and the pressure falls. The
question which will naturally be asked by a beginner at this
point is — supposing the initial pressure is 75 lbs., and the cut-off
•65 of the stroke, how am I to know what the average pressure
of steam will have been by the time the piston has reached the
end of the stroke? Well, if the steam behaved like a perfect
gas, and expanded adiabatically — i.e., without receiving heat from,
or giving up heat to, the cylinder walls — the answer would be

98 MECHANICAL ENGINEERING FOR BEGINNERS.

simple, for, according to Boyle's law, the volume of a gas varies
inversely as its pressure, the temperature being kept constant.
Thus if a given quantity of gas expands to twice its volume, its
pressure falls to half the original pressure ; if expanded to four
times its original volume, the pressure falls to one-quarter of what
it was originally. This law is expressed thus, P x V = constant,
for if we have gas at 100 lbs. pressure, the volume of which is,
let us say, 4 cubic feet, and we multiply the pressure by the
volume, tiie answer is 400 ; if we expand the same quantity of
gas into a space of 8 cubic feet, its pressure will fall to 50 lbs.,
but multiplying the pressure and volume together, we still get
400 as the answer. Therefore, pressure multiplied by volume
equals a constant. This fall of pressure is represented by a
hyperbolic curve, which the reader will be shown how to construct.

The case of steam is not so simple as that of a perfect gas, but
it follows approximately the same law."^ The true adiabatic
curve for steam which is initially dry, falls slightly below the
hyperbolic curve, but the effect of initial condensation and re-
evaporation (referred to later), causes the expansion line of
diagrams taken from actual engines, to approximate closely
to the hyperbolic curve. To construct this curve proceed as
follows : —

Let A B (Fig. 39) represent the stroke of the engine, also the
line of absolute vacuum, and A C the absolute pressure of the
steam (the meaning of absolute pressure was explained on p. 47).
Complete the rectangle as shown, which will then represent the
volume of the cylinder. Mark D at the point of cut-off, and
draw the line D E. Divide E H into equal divisions, 1, 2, 3,
&c., and let fall perpendiculars to the line A B. Draw lines
from the point A to the points 1, 2, 3, 4, <fec., and from the
points where these lines cut E D at a?, x, x draw horizontals
cutting the perpendiculars 1, 2, 3, &c., at the points z, z, z; then
the curve E M drawn through these points is a hyperbola. The
atmospheric line is drawn 14*7 lbs. above the zero line.

Fig. 39 shows how the steam pressure falls if the cut-off takes
place at '25 of the stroke. Figs. 40 and 41 show the fall of
pressure when the cut-oft' takes place at '5 and -75 respectively.

In constructing the above diagrams we have neglected the
effect of clearance — i.e., the steam passages and space between
the piston and cylinder cover when the piston is at the end of
its stroke. If these clearances are to be taken into account,

* The expression for the relation between the pressure and volume of
saturated steam is P x V* = a constant, where the value of x depends on
the dryness of the steam. The expression, P VfJ^ = constant, is frequently
used for saturated steam.

THE STEAM ENGINE.

their volume must be represented by the rectangle shown by
dotted lines in Fig. 41, the point A being set back accordingly.

From a hyperbolic curve it is easy to find the mean pressure
theoretically exerted by the steam throughout its stroke,

Fig. H9.

Zero Line

Fig. 40.

^ M-

I ^^

k\i—

Fig. 41.

assuming P V = constant. Tables giving the mean pressure,
obtained with various points of cut-off, are given in many
engineering pocket-books aild text-books, but the mean pressures

100

MECHANICAL ENOINEERINO FOB BEGINNERS.

SO given are, as a rule, much higher than are found in actual
practice.

In the diagrams given it is assumed that the steam continues
to enter the cylinder at its full initial pressure right up to the
point of cut-off, but in real engines, especially in small fast-
running engines, the steam gets throttled in the passages, and
there is a falling-off in prtssure before cut-off takes place. Again
the diagrams, as shown, assume the steam to be passed away
without causing any back pressure ; also the effect of compression
is not taken into account. In the indicator diagrams taken
from an actual engine, the mean pressure which is really exerted
by the steam is seen, and the data obtained from such diagrams
usually serve as a guide when designing a new engine. The
following Table may be instructive. Column A gives the mean
pressures which should be obtained, according to a well-known
engineering pocket-book, with various cut-offs, and with an
initial steam pressure of 80 lbs. ; column B gives the mean
pressures which should be obtained, according to rules given in
a good book on the steam engine ; while column C gives the
mean pressures which were actually obtained in a good high-
speed engine, indicating about 40 indicated horse-power at full
load.

Initial Pressure 80 Lbs. above Atmosphere.

Point of
Cut-oflf.

A.
Mean

B.
Mean

C.
Mean

Pressure.

•25

47-7

•375

59-5

•5

67-7

53^

•625

73-5

•75

77-3

It is, of course, quite possible that the mean pressure given
in column B might be obtained in a slow-speed engine having
steam passages of ample size, but those given in column A could
only be obtained in a theoretically perfect engine, and must be
treated accordingly.

We have seen how the horse-power of an engine is worked

out, if the mean pressure and all the other factors are known.

If we wish to know what the mean pressure must be to give a

certain power when the speed and other particulars are known>

^, - I . x> 33,000 X HP.
the formula is — P = — o qt? a '

THE STEAM EKGINS. 101

If the speed, mean pressure, and length of stroke are known,
and we wish to find out what area the piston must have in order

to give a certain horse-power, the formula is — A = — ' o q-pp *

The formula is the same in each case, but transposed.

For working out the power of a single-acting engine — i.e., one
in which the steam acts on one side of the piston only — the

formula is-H.P. = ^?.

As already explained, one horse-power is the power required
to raise 33,000 lbs. 1 foot high in one minute, or 1 lb. 33,000 feet
high in one minute.

We have said that the single-cylinder engine is uneconomical.
Why is this, and can it be made to work economically ?

In the first place, with the D-form of slide valve, or with a
single piston valve, the point of cut-off cannot well be made
earlier than § of the stroke, so that when the piston has com-
pleted its stroke a large quantity of steam at a fairly high
pressure, and capable of doing further work, is passed away to
the atmosphere, and is lost, or, in other words, so much heat is
wasted (see Fig. 41). But suppose, instead of the D- valve, we
take the case of an engine fitted with the Corliss or Drop type
of valve, which will allow the cut-off to take place at any part
of the stroke without interfering with the exhaust, and make
the cut-off take place at | or J of the stroke, thus allowing the
steam to work expansively (see Fig. 39). Would this not be
economical ^

It would not, for these reasons. If we cut off the admission
at, say, ^ of the stroke, and expand the steam until the end of
the stroke, the steam, when it leaves the cylinder, is much
cooler than it was when it entered ;. it then cools down the
ports, cylinder walls, and piston, and a certain proportion of
the incoming steam at the next stroke on coming in contact
with the ports, cool walls, and piston, condenses and turns into
water. This is called initial condensation, and is responsible
for a considerable loss of heat or energy.*

* The loss of heat from the cylinder walls is due not only to the fact of
their having been in contact with the cooler steam, but also to the circum-
stance that when the steam has condensed on the cylinder walls it remains
there as a film of water. , Now, after cut-off takes place, the steam expands
and the pressure falls; tKe.film gf water is then in a position to evaporate.
It will have been gathered from the chapter on *' Steam Raising " that the
temperature at which steam is formed depends on the pressure. Now, the
pressure having fallen, the water robs the walls of the heat which had
previously been given to them, and re-evaporates. This loss has to be
ma<ie good by the incoming steam at the next strobe.

102 MECHANICAL ENGIKEEBINO FOR BEGINNERS.

From data in his possession, the author finds that a good
single-cylinder non-condensing engine of the high-speed type,
working with 75 lbs. initial gauge pressure and '7 cut-oflF, used
31 J lbs. of steam per I.H.P. per hour; while with the same
pressure and an extremely early cut-off — viz., '2 — the engine
used 32 lbs., so that the early cut-off was actually harmful.
The best point of cut-off for this engine was '5, when it used
29^ lbs. of steam per I.H.P. per hour. It should be remem-
bered, too, that a good high-speed engine suffers less from initial
condensation than a slow-speed engine, as in the latter there is
more time for the interchange of heat to take place between
the periods of admission.

This initial condensation prevents one from obtaining much
advantage from taking the exhaust steam from a single-cylinder
engine to a condenser, as the ports and cylinder walls are
subjected to a still lower temperature than when non-
condensing.

Cushioning in a single-cylinder non-condensing engine, and
in the H.P. cylinder of a compound condensing engine has a
beneficial effect on the economy of the engine, as not only is
a certain volume of steam saved, but also the temperature of the
compressed steam is raised, and the initial condensation of the
incoming steam is reduced.

To use steam to its greatest advantage we need to have it at
as high a temperature as possible (within limits), to make it do
as much useful work as possible, during which time it will be
giving up its heat, and then to get rid of it without cooling
down the incoming steam more than is necessary. The greatest
ideal efficiency of a steam engine is

Tl - T2 .
Tl '
where Tl = temperature of steam supplied.
T2 = „ „ rejected.

The conditions just mentioned can be more nearly complied
with in a steam turbine than in a reciprocating engine. To
approach them in the latter we need two, three, or even four
cylinders.

A compound engine is one in which the expansion of the
steam is carried out in a pair, or pairs of cylinders. In a two-
cylinder compound engine there is one high-pressure (H.P.)
cylinder and one low-pressure (L.P.) cylinder. The steam, after
doing its work in the H.P. cylinder, instead of being passed
away to the atmosphere or condenser, is taken to a recep-
tacle or receiver, and from this receiver the L.P, cylinder

THE STEAM ENGINE. 103

draws its steam. The L.P. cylinder is much larger in diameter
than the H.P. cylinder, the area of the L.P. piston being usually
three or three and a half times greater than that of the H.P.
piston. As soon as the valve of the L.P. cylinder opens, steam
enters from the receiver and continues to do so until the cut-off
takes place, when the steam expands and forces the piston to
the end of its stroke; the exhaust port then opens, and the
steam is either passed to the atmosphere or to the condenser.
Thus the H.P. cylinder is never cooled down by the com-
paratively low final temperature of the out-going steam.

When the steam is exhausted to the open air it has to be
discharged against the pressure of the atmosphere, which is
about 14*7 lbs. (temperature of the steam, 212® F.). If, however,
the steam is condensed by means of cold water, a vacuum is
formed, and the steam is discharged against an absolute pressure
of 1 or 2 lbs. only (the exact pressure depends on the condensing
arrangements and size of the exhaust pipe), and at a temperature
of 100° to 125**. Thus, more work is got out of the steam than
can be obtained when discharging against atmospheric pressure.

In the earliest form of steam engine steam was used for
raising the piston only. When the piston reached the top of its.
stroke a jet of water was squirted in, a vacuum was created, and
the atmospheric pressure forced the piston down. The initial
condensation must, of course, have been enormous.

In a triple-expansion engine the expansion of the steam is
carried out in three cylinders ; in such an engine the range of
temperature in each cylinder is still less than in a compound
engine, and the loss from initial condensation is therefore still
further reduced.

The consumption of steam, not superheated, in a good compound
engine of the Corliss type, when condensing, is between 14 and
15 lbs. of steam per I.H.P. per hour. When exhausting to the
atmosphere the consumption is between 19 and 22 lbs. per I.H.P.
per hour.

The consumption of steam in a good triple-expansion engine
when condensing is between 12^ and 13.V lbs. per I.H.P. per
hour. When exhausting to the atmosphere the consumption is
between 18| and 19 J lbs. per I.H.P. per hour, assuming the
steam pressure is not less than 180 lbs. above the atmosphere.
With steam pressures lower than this there is but little advan-
tage in employing a triple-expansion engine, when it has to
work non-condensing.

The consumption of steam in a quadruple-expansion engine is
slightly less per I.H.P. than in a triple-expansion engine, but it
is somewhat doubtful whether the gain through carrying out the

104 MECHANICAL ENGINEERING FOR BEGINNERS.

expansion in four stages instead of three is sufficiently great to
compensate for the extra first cost, and for the extra friction due
to the use of the fourth cylinder.

In marine engines, which are usually tested for economy after
erection on board ship, the economy or otherwise is usually
referred to in lbs. of coal, as it is rather a troublesome matter to
weigh or measure the water used. The average consumption of
coal on modem battle-ships having triple-expansion engines is
1*78 lbs. of coal per I.H.P. per hour at the most economical load
— viz , about 70 per cent, of the full power, and 1*92 lbs. of coal'
at full power.

In the Britannia^ where superheated steam was used, the con-
sumption of coal was 1*5 and 1*85 lbs. respectively.

These consumptions of c^al include the amount burnt for
making steam for the auxiliary machinery. In the mercantile
marine, where there is less auxiliary machinery, the consumption
of coal is from 1-3 to 1*75 lbs. per I.H.P. per hour.

The consumption of coal in locomotive engines, as tested in
America by means of a dynamometer, varied from 3*5 to 4 5 lbs.
per B.H.P. in simple engines for goods traffic, and from 2 to
3*7 lbs. in compound engines for the same class of work. The
consumption of coal in simple passenger locomotives varied from
3*5 to 5*0 lbs., and in compound passenger locomotives from
2*2 to 5*0 lbs., the higher consumption of coal always occurring
at high speeds.

In a mill having a good compound condensing engine a con-
sumption of 1*5 to 1*75 lbs. of coal per I.H.P. is regarded as a
very fair performance.

Corliss Gear. — We have already spoken of an engine fitted
with the Corliss type of valve gear, by means of which the cut-
off can be made early without interfering with the opening of
the exhaust port at the proper time. Fig. 42 shows the Spencer-
Inglis form of this gear in elevation, and Fig. 43 shows a section
through the L.P. cylinder of a large Corliss engine recently
supplied by Messrs. Fullerton, Hodgart & Barclay, of Paisley,
for the East Randt Gold Mining Company.

The Corliss form of valve gear was invented by an American
engineer, whose name it bears. It has been very largely used
in England and America, but is only suitable for engines running
at a speed of 120 revolutions per minute or less.

It will be seen from Figs. 42 and 43 that there are separate
admission and exhaust valves at each end of the cylinder ; by
this arrangement long steam ports are avoided, and the relatively
cool exhaust steam does not pass through the axlmission ports, as
is the case with an engine having a single D-slide valve. The

THE STEAM ENGINE.

105

exhaust ports, which are at the lowest portion of the cylinder,
allow any water which has not re-evaporated to be swept out by
the piston at every stroke.

B_B

Fig. 43. — L.P. cylinder, with double-ported Corliss valves.

The Corliss gear may perhaps look complicated to the beginner,
but really it is very simple. The steam-admission valves at the

106 MECHANICAL ENGINEERING FOR BEGINNERS.

top of the cylinder are kept closed by the rods A A, Fig. 42.
Each of the rods is connected to a plunger inside the small
cylinders and dashpots placed back to back at BB. The plungers
usually are drawn in by means of springs, and, less frequently,
by placing one side of the plunger in communication with the
condenser, and leaving the other side open to the atmosphere.
The effect is to keep the main steam-aidmission valves closed
until one of the blocks, C, which is connected to a lever fixed to
the valve spindle, is taken hold of by the catch plates D D ; the
motion of these catch plates and the block they have caught
hold of causes the steam valve to open. When the piston has
reached a certain position the catch plates are opened outwards,
the block C is released, and the main steam valve immediately
closes.

The mechanism which causes the plates to open outwards is
shown by the inset. The releasing toe is pivoted to a prolonga-
tion of the block C already referred to, and when drawn along
with it — the end of the lever attached to the toe being prevented
from moving inwards by the rod R — the toe is forced to take
up a more oblique position across the catch plates, and opens
them outwards. The rod R, is also connected to the governor,
so that if the engine runs too fast the toe is forced into its
oblique position a little earlier in the stroke ; the point of cut-off
is thus regulated by the governor.

The catch plates D are worked from the wrist-plate F, which
receives its oscillating motion from an eccentric, or eccentrics, in
the ordinary way. The exhaust valves have no trip gear like
the steam valves ; they are connected by the rods H H to a
second wrist-plate placed behind the first, and are unaffected
by the action of the governor. Sometimes both steam and
exhaust valves are worked off the same wrist-plate, but a
greater range and more accurate setting can be obtained with
two wrist-plates.

The sectional view (Fig. 43) is almost self-explanator. The
cylinder shown is 40 inches in diameter, has a 5 feet stroke, and
is steam- jacketed. The speed is 51 revolutions per minute.
The valves are double ported. Steam from the H.P. cylinder
enters at the far side, and passes up the passage, which forms
part of the receiver, shown by dotted lines. The entrance to
this passage is beyond the exhaust opening, and is, of course,
separated from it. The steam valve at the top right-hand end
has been drawn open by the catch plates, while the steam valve
at the top left-hand end, which had previously been tripped, is
closed. The exhaust valve at the bottom right-hand end is
closed, while the exhaust valve at the other end is open. The

THE STEAM ENGINE.

107

opening K is for the purpose of draining the jacket. The
admission to the jacket is at the far side of the cylinder, and is
not shown. The glands of the piston and tail rods are packed
with a patent metallic packing, which is described later.

Fig. 44.— Drop valve engine.

Fig. 44a. — Van der Kerchove engine.

108 MECHAKICAL ENGINEERING FOR BEGINNERS.

Drop, or Double-beat ValTes. — Another form of valve,
which is considered by some engineers to be more suitable for
use with superheated steam than the Corliss valve, is the double-
beat valve. The L.P. cylinder of an engine fitted with these
valves is shown by Fig. 44. The valves, which are of the
equilibrium type, are usually worked by some form of trip gear
actuated by eccentrics placed on a shaft running alongside the
cylinder, and parallel to the piston-rod. A considerable number
of engines fitted with this form of valve has been made in this
country and abroad. The chief objection to them is that the
valves close with a somewhat heavy blow, causing wear and
tear, and resulting in leaky valve seatings. Another objection
is that the clearances are rather greater than is the case with
the Corliss form of valve. To overcome these objections Messrs.
Van der Kerchove, a well-known firm of Belgian engineers,
have brought out and patented the engine shown by Fig. 44a,
in which piston valves are used instead of double-beat valves.
The piston valves are worked by trip gear ; they are provided
with piston rings and work, of course, in liners in which ports
are cut. The piston valves have lap, so that the cushioning of
the valve is effected after it has closed the port. The clearances,
as will be seen from the illustration, are extremely small. This
type of engine is made in England by Messrs. Musgrave, of
Bolton.

The Lentz valve gear consists of valves of the double-beat
type, shown by Fig. 44. The valves are, however, not
'forked by any form of trip gear, but take their motion from
a rocking cam actuated by eccentrics, and the usual link-
reversing gear.

The Willans Central-valve Engine. — An engine of an
entirely different type, and one which has had a great vogue in
electric generating stations in this country, is the high-speed
Willans central- valve engine, illustrated diagrammatically by
Fig. 45. The illustration shows a three-crank triple-expansion
engine, the action of which is briefly this — Steam enters the
steam chest A, and passes through ports cut in the trunk or
hollow piston-rod T, when the latter is at the top of its stroke.
Moving up and down inside each trunk is a line of piston valves
worked by an eccentric placed on the crank pin. These piston
valves control the admission of steam to the cylinder, and the
exhaust from the upper to the lower side of the piston, the space
below each piston being the receiver R. The steam, after having
done its work in the H.P. cylinder B, is passed to the underside
of the piston or receiver R. From this receiver the inter-
mediate cylinder D draws its steam, and in turn passes it on

THE STEAM ENGINE.

109

to its receiver R^, and thence to the L.P. cylinder E; after
leaving the L.P. cylinder, the steam enters the exhaust chamber
G, which is placed in communication either with the atmos-
phere or with the condenser.

Fig. 45. — Triple-expansion three-crank Willans engine.

A, Steam chest. U, Air chamber.

B, H.P. cylinder. K, Guide piston.?
R, H.P. receiver. T, Trunks.

D, Intermediate cylinder. L, Eccentrics and straps.
R, Intermediate receiver. M, Connecting-rods.

E, L.P. cylinder. P, Piston.
O, Exhaust chamber.

The Willans engine is single-acting — i.e., the steam acts on one
side of the piston .only, so that there is no push-and-pull action
on the brasses. In order to prevent a pull coming on the
brasses on the up-stroke due to the momentum, or more scienti-

110

MECHANICAL ENGINEERING FOR BEGINNERS.

fically expressed, to the inertia * of the moving parts, a separate
chamber H is provided, in which air is compressed during the
upward stroke, the greater portion of the power expended in
compressing the air being given out on the downward stroke.
The piston K, which compresses the air, serves also as the cross-
head and guide. The crank chamber below the crank shaft is
partly filled with oil, which is splashed up over the journals and
into the guide cylinders.

The horse-power of some standard Willans engines and the
speeds at which they run are as follows : —

Table XV.

Makers'
Distinguishing

LH.P.

Revolutions per
Minute.

90 to 100

470

120 „ 150

460

200,, 240

380

300,, 350

350

400 „ 450

340

500 „ 575

320

600,, 700

300

750 „ 825

270

1,000 „ 1,250

230

2,000 „ 2,500

200

Although the number of revolutions at which the Willans
engine runs is very high, yet the piston speed, owing to the
short stroke, is comparatively low. The piston speed ranges
from 470 feet per minute in the case of the 100 I.H.P.
engine to 787 feet in the case of the largest engine of all.
These speeds compare favourably with those of many large
marine engines and locomotives ; for instance, the piston speed
of the engines of H.M.S. Africa is 1,124 feet per minute when
running at full speed — viz., 128 revolutions per minute; and 920
feet when running at cruising speed — viz., 115 revolutions.
The piston speed in a modern locomotive having cylinders
19 J inches diameter by 26 inches stroke and 6-feet 6-inch driving-
wheels, is 1120 feet when running at 60 miles per hour. The

* It should be explained that a body requires a certain force to set it in
motion ; when once set in motion the body will continue to move in a
straight line until stopped by gravity or by some other force. This
unwillingness to start and unwillingness to stop when once set going is
called the inertia of the body.

THE STEAM ENGINE. Ill

piston speeds of large horizontal Corliss engines are usually about
600 feet per minute.

Belliss and Browett-Iiindley Engines. — Two other well-
known high-speed engines are those of Messrs. Belliss and
Browett-Lindley. These engines are double-acting, the principle
being the same as that of the engine shown by Fig. 35. The
engines, however, are usually made either compound or triple-
expansion. The compound engine has one high-pressure and
one low-pressure cylinder placed side by side and two cranks ;
the triple engine has three cylinders placed side by side and
three cranks. It may be here remarked that a three-crank
engine, especially if the pistons are approximately of the same
weight, causes much less vibration than any two-crank engine ;
for this reason, three-crank engines are usually selected for those
electric generating stations which are surrounded by dwelling-
houses. The vibrations from two -crank high-speed engines,
even if mounted on heavy foundations, are sometimes trans-
mitted to considerable distances, especially if the soil is moist,
and may cause considerable annoyance to residents during the
night time. The vibrations set up by a slow-speed engine, as
shown by a vibration recording instrument, may be greater than
those of a high-speed engine, but the period of vibration being
slower the annoyance caused is not so great.

Both the Belliss and Browett engines are forced lubrication
engines — i.e., the lubricant is forced into the bearings under
pressure, a pump driven off the crank shaft being provided for
the purpose. The powers and speeds of these engines are
approximately the same as those of the Willans engine already
given. It is owing to the short stroke of these engines and to
the system of forced lubrication that they are able to run at
such high rotative speeds.

A few words as to the merits of each type of engine, and as to
the disadvantages connected with its use, may now be said.

Slow-speed Horizontal Engines. — These engines are very
largely used in mills and works of all kinds where fair economy,
freedom from trouble and breakdown, and a long life are the
most important considerations. The engines usually found in
mills are of the compound type. A compound engine is not
quite so economical as one of the triple-expansion form, but the
former works with very fair economy. The consumption of dry,
but not superheated, steam when condensing is about 14J lbs.
per I.H.P. per hour. The wear and tear of such engines, if
well made and properly lubricated, is very small, and the engine
requires but little attention.

The fact of the heavy pistons rubbing on the lower half of the

112 MECHANICAL EKGIVEERiyG FOR BEGINNERS.

cylinder is not a good feature, and for many years past it has
been the custom to prolong the piston-rod, so as to pass through
the back cover, and provide it with a block running on a slide,
the idea being to carry the weight of the piston. A circular
rod, however, is almost the worst form that a beam designed to
carry a weight can take, and it is now considered by many
engineers that it is better to make the piston of great width, so
as to provide ample bearing surface, and to dispense with the
tail rod. It is claimed that the wear of a cylinder having a
wide and light piston is more even, and less in extent, than is
the case with a cylinder having a narrow piston supported by
a tail-rod which sags. In any case a cylinder, the liner of which
is made of good hard cast iron, and has been truly bored in the
first place, will last for a great many years without requiring to
be re-bored.

The reason why horizontal engines are not employed in large
electric generating stations is because they cannot run at speeds
suflSciently high to enable them to be directly coupled to the
dynamos, and the space required for driving the latter by ropes
cannot be given. In any case the use of ropes, involving a loss
of 5 per cent, or 7 per cent, would not be tolerated.

Vertical Slow-speed Engines. — These are employed to the
exclusion of almost all other types of reciprocating engine for
marine work. They occupy less floor space than a horizontal
engine, but are more expensive to construct. The frame for
carrying the cylinders requires to be much heavier and stronger
than is necessary in the case of a horizontal engine, the cylinders
of which in land work rest on concrete. Platforms and ladders,
too, are required with vei*tical engines, thus adding to the
expense.

A good feature about vertical engines is that there is not the
same risk of the cylinders wearing oval as in a horizontal engine,
as the piston does not bear upon the cylinder walls. A vertical
engine lends itself more readily to the triple-expansion form
than a horizontal engine, and in some mills and works where
first cost is of less importance than extreme economy of steam,
vertical triple -expansion engines may be found. Vertical
engines of the marine type are used in some electric generating
stations, but their comparatively slow speed necessitates a very
large and costly generator if the latter is to be coupled direct to
the engine.

High-speed Single-acting Engines. — The Willans central-
valve single-acting engine was introduced at the time when
electric lighting was first being carried out upon a considerable
scale, and when it was found necessary to install as much power

THB STHAM ENGINE. 113

as possible in a small space. The high speed of the Willans
engine enabled it to be coupled direct to the dynamo ; thus the
space which would otherwise have been required for a belt or
rope drive was saved, also the loss due to this method of trans-
mission was avoided. In addition, the engine was remarkably
eoonomieal in steam, and was capable of making very long runs
without a stop. It is on record that one of these engines ran for
seven months night and day in a copper depositing works without
a single stop. The success of the Willans engine in important
generating stations, such as those of the St. James and Pall Mall
Company, and of the Westminster Company, led to its being
widely adopted all over the country, even in places where ample
space was available, and for many years this engine practically
held the field for electric lighting and traction work. In addition
to the advantages of high speed and economy, one other must be
referred to. The engine is single-acting, and, owing to the
action of the air buffers (already referred to), the constant thrust
principle is really carried out. As there is no push-and-puU
action, wear of the brasses does not give rise to a knock, and,
within limits, is unimportant. The engine, therefore, does not
require frequently to be laid off, and a mechanic's time occupied
in setting up and adjusting brasses.

The Willans engine has been employed in a few cases for
driving cotton and flax mills, for which purpose the large
number of impulses per minute and consequent even turning
render it very suitable. It is stated that the output of a flax
mill in Ireland was increased to the extent of 5 per cent,
through freedom from breakages of thread, by the substi-
tution of a Willans three-crank engine for one of the slow-speed
type.

The disadvantages of a single-acting engine are as follows : —
The piston must have an area twice as great as that required in
a double-acting engine of equal power, running at the same
speed. If a knock is heard in a single-acting engine, such as
the " Willans,'' it is advisable to take the engine down at once
to ascert tin the cause, for in the Willans engine the pistons are
mounted on cast-iron trunks, through which ports are out, and
through which steam is admitted to the cylinders. If a broken
piston ring or valve ring should get into one of these ports while
the engine is running, it might cause a very serious breakdown.
As a fact, however, breakdowns of this nature are comparatively
rare.

In the Willans engine, too, there is a constant, though small,
loss due to compressing and expanding the air in the air
chambers. This loss prevents quite such a high mechanical

114 MECHANICAL ENGINEERING FOR BEGINNERS.

efficiency being obtained, as in a double-acting engine, in which
there are no air buflfers.*

Double-acting High-speed Engines. — The success of the
Willans single-acting engine incited makers of double-acting
engines to construct them to run at speeds approximating to
those of the former, and by extremely good workmanship and
by the use of forced lubrication, they succeeded. The best
known high-speed double-acting engines are those made by
Messrs. Belliss and by Messrs. Browett & lindley.

The advantages of these engines for driving dynamos are
those possessed by the Willans engine, and the engines, con-
forming more nearly as they do to the ordinary type of slow
speed engines, can be overhauled by men who have had no
special training. The solid steel piston-rod in these engines is
preferred by many to the cast-iron trunks in the Willans engine.

For electric generating purposes a high-speed engine has in
the past been a necessity, but its place is seriously threatened
by the advent of the steam turbine. For mill work, and in
cases where high speed is not a necessity, engines of the long-
stroke slow-running type are likely to hold their own against
their lighter and quicker running rivals.

Steam Jacketing. — The practical advantages or otherwise of
jacketing steam cylinders — i.e., keeping them surrounded by live
steam — has been under discussion for a great many years past.
It is claimed that by jacketing a cylinder and imparting heat to
its walls while expansion is taking place, and during exhaust,
the (all in temperature of the walls is reduced and initial con-
densation is lessened. On the other hand, a certain proportion
of the steam in the jacket is condensed, and opponents of jacket-
ing say that as much steam is lost in the jacket as is saved in the
cylinder.

Experiments appear to show that there is a gain through

* If this little book should fall into the hands of a station engineer, who
would like to ascertain the loss of efficiency due to the air buffers, and who
has been unable to ascertain the loss by means of an indicator card, he can
easily do so, assuming the engine is coupled to a direct current dynamo,
by the following method : — The first time the engine is down for any
purpose, remove the steam cylinders, pistons, and high-presimre trunks,
leaving the guide piston, low-pressure trunks, and air buffer covers in
position. Run the dynamo as a motor, and note the current required to
drive the crank shaft at its full speed, with the guide pistons and low-
pressure trunk running — i.e., with the buffers in action. Then remove the
guide pistons, and note what current is required to run the crank shaft at
full speed — i.e., with the buffers out of action. The difference of current
(neglecting the small difference of dynamo efficiency) will show the loss
due to compressing the air in the buflfers. The loss is not truly shown by
an indicator card.

THE STEAM ENGINE. 115

jacketing, if properly carried out. This gain is probably due to
the fact that it is better to condense the steam outside the
cylinder than inside it, as it is the film of water inside the
cylinder walls which enables the transfer of heat to take place
so easily, and which robs them of so much heat when re-evapor-
ation takes place. Whether the gain in actual practice is as
great as in experimental trials is, however, open to doubt, for it
must not be overlooked that for the jacket to be effective the
condensed steam must be got rid of, and those who have had
some experience in connection with steam traps know that they
have a tendency either to stick, or to leak. If the trap sticks
the advantage of the jacket is lost, and if the trap leaks it is
quite possible that the steam lost in this way may equal that
saved in the cylinder.

There is, however, one advantage in jacketing an engine
which the maker cannot afford to overlook — it makes a good
selling point. If A's representative can say to an intending
purchaser, "My engine is jacketed, while B's is not," it may
possibly turn the scale in A's favour ; and it must be remem-
bered that, distasteful as the fact may appear, the majority of
engine builders are in business, not for the pleasure of the thing,
nor for the purpose of contributing papers to learned societies,
but to make money for themselves, or for their shareholders.

Calculating the Power of Compound Engines. — The
reader has seen how to work out the power of a single cylinder,
or simple engine ; to calculate the horse-power of a compound or
triple-expansion engine, the formula for the simple engine holds
good ; but if the student has not got the actual indicator dia-
grams before him, care must be taken to estimate the mean
pressures rightly. With condensing engines it is also more
convenient to make the calculation with absolute pressures — i,e,,
including the atmosphere. Instead of saying 75 lbs. above the
atmosphere we should say 89*7 lbs. absolute, or, roughly, 90 lbs.
absolute.

We will now take the case of a double-acting compound con-
densing engine, having one H.P. cylinder 10 inches diameter
and one L.P. cylinder 18 inches diameter, the stroke of each 8
inches. The speed, 300 revolutions per minute. In working
out the power of the simple engine with approximately 90 lbs.
absolute pressure, we assumed the mean pressure to be about
65 lbs. absolute. If, however, instead of passing the exhaust
steam direct to the atmosphere, we take it to another and larger
cylinder to do more work, the pressure of the steam acting on
the large L.P. piston will also exert a back pressure upon the
small H.P. piston, so that, instead of having a mean effective

116 MECHANICAL ENQINEBRIN6 FOR BEGINNERS.

pressure of 65 lbs. absolute on the H.P. piston, we shall only
have an effective mean pressure of about 40 lbs. The mean
pressure in the L.P. cylinder will, of course, be much less,
as its area is about 3^ times greater than that of the H.P.
cylinder, and it only receives its steam after this leaves the H.P.
cylinder. If in this case we cut off at -66 of the stroke, we shall
expand the steam about 5 times altogether. The effective mean
pressure we might get in the L.P. cylinder would be about 20
lbs. absolute. We should, therefore, have to take the area of the
10-inch piston — viz., 78*5 inches — as having an effective pres-
sure of 40 lbs., and the area of the 18-inoh piston — viz.,
254*4 inches — as having an effective pressure of 20 lbs. The
result would be— 78-5 x 40 = 3,140 lbs., and 254-4x20 = 5,088
lbs., or 8,228 lbs. total effective pressure on the two pistons.
For the sake of simplicity in the calculations, it is usual to refer
this total pressure to a corresponding pressure on the L,P. piston.
In this case the area of the L.P. piston is 254*4 inches ; and if
we divide the total effective pressure on the two pistons by this
area, we shall have the corresponding effective pressure on the
L.P. piston. Thus, 8,228 -4- 254*4 = 32*3 lbs. mean pressure, so
that in our calculations for horse-power we may ignore the H.P.
piston, and take into account the L.P. piston only, with the
corrected mean pressure referred to it.

The formula for horse-power, as stated before, is »» ^^^

The calculation for the compound engine will, therefore, be —
2 X -666 X 300 X 254*4 x 32*3 ^o k t tj t>

spoo == ^^'^ ^•^•■^•

Testing Steam Engines. — There are two methods of ascer-
taining how much steam an engine uses. The first, by weighing
the water pumped into the boiler, and the second, by weighing
the steam (condensed) as it leaves the engine. The first method
is usually adopted when it is desired to make a test after an
engine has been fixed at the site and is doing actual work ; but,
unless precautions are taken, this method frequently makes the
engine appear to use more steam than is really the case. For
instance, should there be a small leak in the furnace of the
boiler, any water which escapes is immediately turned into
steam, and probably passes away with the furnace gases
unnoticed.

To avoid such a loss, it is advisable to make a " still " test of
the boiler over a period of 10 or 12 hours. For this test, steam
is raised in the boiler to within 5 or 6 lbs. of the pressure at
which the safety valves blow off. The level of the water in the

THE STEAM ENGINE. 117

boiler is carefully observed, all valves are then closed (the steam
pipe should be closed with a blank flange), and steam maintained
at a pressure below blow-off pressure. At the end of 10 or
12 hours, water which has been weighed is pumped into the
boiler until the original level is reached. This weight of
water is the amount of boiler leakage during the 10 or 12
hours' test. The leakage is often considerable, even in a
boiler which has appeared to be tight when tested under
hydraulic pressure.

In carrying out an engine trial by this method, care must be
taken to see that during the trial the boiler safety valve does
not blow off, that there are no leaks of steam from the steam
pipe connecting the boiler to the engine, and that no auxiliary
machinery, such as a separate donkey feed pump, is supplied
from the boiler into which water is being weighed.

The second method is more accurate so far as the engine is
concerned, and is that usually adopted at the works of engine-
makers who are in the habit of testing their engines before
delivery. By this method the steam as it leaves the engine is
condensed and passed into a tank, which is placed on a weighing
machine. It is certain that all steam which goes into an engine
must come out of it, otherwise the engine would become choked
with water.

If the engine is ultimately to work non-condensing, and non-
condensing results are required, the chamber in which the
steam is condensed is left open to the atmosphere, so that there
is no vacuum. By this system of measurement boiler leaks are
immaterial, as the engine is only debited with the exhaust steam
which comes out of it. The boiler, too, may drive any auxiliary
machinery while the test is being made.

A test of one or two hours' duration made in this way is more
accurate than a 10 or 12 hours' test made by the method first
described, as readings can be taken every few minutes, and if
the readings tally with one another, the result may be relied
upon implicitly.

To take an actual case. Suppose an engine which is giving
100 horse-power passes regularly every minute 25 lbs. of water
into the tank, we know that the engine is using 1,500 lbs. of
steam per hour. If now we divide this total water by the
horse-power, we see that the engine is using 15 lbs. of steam per
horse-power per hour. It is not necessary to have a tank of
very large proportions, for, if the readings tally, it does not
matter how often the tank is emptied. The arm of the weigh-
bridge upon which the tank is placed is usually arranged so that
as soon as a given quantity of water has passed from the engine

118 MECHANICAL ENGINEERING FOR BEGINNERS.

into the tank the arm rises, the observer notes the time and
pushes forward a weight which depresses the arm; as soon as
the given quantity of water has again entered the tank, the arm
, again rises and the observer again notes the time, pushes forward
the weight, and so on.

Having got the boiler, engine, and weighing apparatus, how is
a load to be provided for the engine, assuming we wish to test it
before it leaves the maker's works ?

If the engine is coupled direct to a dynamo the matter is
comparatively easy; all that is necessary is to provide resistances
in the form of wires through which the current will have to
travel ; the power of the engine will be absorbed in generating
the electric current^ and the current will be dissipated in the
form of heat in passing through the wires of low conductivity.
If suitable wire resistances are not available, a resistance can be
formed by immersing two metal plates in a tank of water, and
making the current force its way through the water between
them. Such a water resistance is, however, not so satisfactory
as a wire resistance, as it is difficult to keep the load steady.
The electrical method of providing a load for the engine is a
good one, as if the volts and amperes given by the dynamo are
noted and its efficiency is known, the actual brake horse-power
given by the engine can be calculated. Chapter xi. will show
how such calculations are made. If, however, the engine is not
coupled to a dynamo, a load can be put on the engine by means
of a brake.

This brake, too, can be made to show how much horse-power
the engine is giving. In this way; we know that 1 horse-
power is the equivalent of lifting 33,000 lbs. 1 foot high in one
minute. Now, if we were to arrange matters so that the
friction caused by a band surrounding a pulley which was
travelling at the rate of 1 foot in one minute, exerted a pull
on the band of 33,000 lbs., this would be equivalent to lifting
33,000 lbs. 1 foot in one minute. A rope-brake working on
this principle is frequently used. It consists of several pieces
of rope arranged to form a flat band (similar to the plaited
leather horse -girths sometimes used when schooling young
horses). The rope is well greased, and is lapped round the
pulley of the engine to be tested, one end is attached to
a portable weighing machine of the " Denison " type, preferably
carried by a crane, the other is attached to a strong spring
balance. Of course, in actual practice the face of the pulley
travels much faster than 1 foot in one minute, and the pull on
the ropes is much less than 33,000 lbs. for every horse-power,
but these figures make the principle clear.

THE STEAM ENGINE. 119

In the case of a brake load, the work done by the engine is
absorbed in friction, and passes away in the form of heat. The
fact that the work done by the engine passes away as heat, does
not seem to be clearly realised by all of those who have to put a
load on an engine by means of a brake. The author knew one
individual who felt distinctly aggrieved because the ropes forming
the brake got too hot and charred, in spite of his having used
tallow, &c. He complained that he had to throw buckets of
water on the brake ropes and pulley to keep them cool, thus
causing a great mess on the floor.

In case the reader would like to see approximately how much
water would be required to be thrown on a rope-brake absorbing
100 H.P., in order to keep it cool, neglecting the dissipation of
heat by radiation and conduction, the method of making the
calculation is explained. It is as follows : — 100 H.P. x 33,000
ft. -lbs. = 3,300,000 ft. -lbs. of energy to be got rid of in one
minute. We read in Chapter iii. that 772 ft.-lbs. are the
equivalent of 1 B.T.U., also that 1 B.T.U. will raise 1 lb. of
water by V F., so that, if we divide 3,300,000 by 772, we shall
see how many thermal units have to be got rid of, and how
much water will be required. 3,300,000 -^ 772 = 4,274 B.T.U.
Assuming the water in the buckets is at a temperature of 60" F.,
and after being thrown on the brake its temperature is raised to
212°F., each lb. of water will take away 152 thermal units, and
a gallon of water (weighing 10 lbs.) will take away 1,520 B.T.U.
Now, if we divide 4,274 by 1,520, the answer is 2*81 gallons of
water per minute, so that 2*81 gallons of water per minute will
require to be thrown on the brake to keep it cool when absorb-
ing 100 H.P. If, however, instead of throwing water on the
brake by the bucketful, and only raising its temperature to 210°
or 212° F., matters could have been arranged so that the water
was thrown on the brake in a fine spray, say from a hose with a
fine rose, and the whole of the water was turned into steam, a
very much smaller quantity of water would have been required,
for we read in Chapter in. that a very much larger quantity of
heat is required to evaporate a lb. of water at 212° than is
required merely to raise it to this temperature. Let us see how
much water would be required to keep the brake cool if all the
water thrown on to it were vaporised. It has already been said
that the latent heat of 1 lb. of steam at 212'' F. is 966 B.T.U.,
therefore the water, if turned into steam, will absorb all this
heat from the brake, as well as the 152 B.T.U. required to raise
the water from 60' to 2 1 2°. Every pound of water thrown on the
brake will, therefore, absorb 966 4- 152, or, say, 1,118 B.T.U.,
and as we have seen that 4,274 B.T.U. must be got rid of per

120 MECHANICAL EN6INEBBING FOR BEGINNERS.

minute, 3-82 lbs., or a little more than ^ of a gallon of water per
minute, will answer the purpose.

A sound and working knowledge of the principles of heat and
steam would have shown the engineer, to whom we have referred,
(1) that the brake was bound to get hot when absorbing power;
and (2) that by using smaller quantities of water, and turning
most, if not all, of it into steam, he could have prevented a good
deal of the mess on the floor.

It will be realised, from the foregoing, that any form of
friction brake, unless suitable means are provided for taking
away the heat^ is quite unsuitable for absorbing large powers.
When dealing with engines of small power, sufficient heat is
lost by radiation to enable the brake to keep fairly cool.

A much more convenient form of brake is the Froude water
brake (constructed by Mather & Piatt, of Manchester). In this
brake there is an internal wheel, carried on a long shaft, having
a coupling at one or both ends ; the wheel consists of two saucer-
like vessels placed back to back, each divided by vanes having a
forward rake. The external casing, which is free to turn on the
shafts has two similar saucer-like vessels, also divided by vanes
having a backward rake. This external casing carries a long
arm or lever, at the end of which weights are suspended. The
engine which is to be tested is coupled to the flange on the shaft
carrying the wheel, and water is admitted to the brake. The
rotation of the wheel acting on the water tends to make the
casing rotate, but it is prevented from doing so by the weights
carried at the end of the arm. The rotary action, however,
causes the arm to rise and lift the weights, and the weights
being known, the actual power can be calculated.

The water passes through the brake in a continuous stream,
the work done being in the form of heat imparted to the water

Such a brake is somewhat expensive, especially if of large size,
and is only possessed by a few engineering Arms, but its value
may be realised from the following incident which came to the
author's knowledge a short time ago. A Arm of engine makers
accepted a contract for some engines, which were guaranteed to
give a certain mechanical efficiency. The engines were duly
constructed, and coupled to some dynamos, which were also
guaranteed to give a certain mechanical efficiency. When the
plant was tested, the combined efficiency was not equal to that
which would have been obtained had the engine maker's and
dynamo maker's guarantees been kept. Perhaps we had better
explain what is meant by combined efficiency. Let us say that
the engine maker guaranteed a mechanical efficiency of 90 per
cent, and the dynamo maker an efficiency of 90 per cent., the

THB STEAM BNOINE.

121

€00ibine(i efficiency of the plant should be 81 per cent. That is
to say, the actual electric output should be 81 H.P. for every
100 H.P. developed in the engine cylinders.

Fig. 46. — Crosby indicator.

Fig. 46a. — Crosby Indicator for superheated steam.

The efficiency in the case in question not being obtained, the
question arose as to who was in fault. The power of the engines
$eing too great to admit of the use of a rope brake, the matter

122 MECHANICAL ENOINEERIKO FOR BEGINNERS.

was settled by the purchase of a Froude brake, when it was
found that the fault lay with the dynamos.

We will assume that all arrangements have now been made for
giving the engine a suitable load; how are we to ascertain exactly
what I.H.P. the engine under test is giving? The stroke of the
engine is known; the revolutions per minute at which the engine
is running can be ascertained by a mechanical speed counter; the
area of the piston is known ; all then we need to know is the
mean effective pressure on the pistons, or what P stands for in
the formula —

2SBAP
33,000 •

The actual mean pressure exerted on the piston of an engine is
found by means of an indicator. An indicator of the Crosby
make is shown by Fig. 46. This instrument consists of a small
barrel, or cylinder, in which a piston works against a spring.
The piston which in a good indicator is nearly frictionless, actu-
ates a pencil at the end of an arm or lever. When the indicator
barrel is placed in communication with the engine cylinder, any
pressure in the latter causes the indicator piston to rise, and the
pencil marks a piece of paper or card. If the card were held
stationary, a vertical line only would be recorded due to the
rising and falling of the indicator piston, but by moving the
card so that the portion under the pencil corresponds with the
position of the engine piston, a continuous line or diagram is
drawn showing the pressure exerted on the engine piston at all
points of the stroke. The drum to which the card is attached is
rotated by means of a cord attached to any reciprocating part of
the engine which gives the required motion.

The Crosby indicator, owing to the lightness of its working
parts and to the accuracy with which it is made, is largely used
for indicating high-speed engines, for which purpose 3ie older
patterns of indicator having heavier parts and a longer stroke
are unsuitable. The latest development of the Crosby indicator
is shown by Fig. 46a, from wbich it will be seen that the spring
against which the indicator piston works is placed outside the
barrel. This is a great improvement, especially in cases where
superheated steam is used, as the temperature of the steam does
not affect the action of the spring. Moreover, when the spring
is placed ouside the barrel, there is no possibility of its causing
the piston to bear unduly on one side of the barrel.

Springs of different strengths are supplied with the indicator.
Thus, one spring will travel upwards 1 inch with a pressure of
40 lbs. in the barrel, while another will travel 1 inch upwards

THE STEAM ENGINE. 123

with a pressure of 10 lbs. One is called a 40-lb. spring, the
other a 1 0-lb. spring. The spring used is noted on the indicator
card by the tester ; a much stronger spring is used for indicating
the H.P. cylinder than for the L.P. cylinder. To ascertain the
pressure shown by a diagram, if a 40-lb. spring had been used, a
boxwood scale graduated 40 lbs. to the inch would be used. If
a 10-lb. spring had been used, a scale graduated 10 lbs. to the
inch would be employed.

The indicator barrel is connected to the cylinder by means of
a short pipe and cocks, ari-anged so that the barrel of the indi-
cator may be shut off from the cylinder, and placed in communi-
cation with the atmosphere; when this is done, if motion is given
to the card, a straight horizontal line is drawn, which is, of
course, the atmospheric line.

Optical Indicators. — For speeds above 500 or 600 revs,
per minute, an indicator with a piston, arm, and pencil, is
unsuitable owing to the inertia of the moving parts. To indicate
petrol or other engines running at very high speeds, optical
indicators are used. In these the piston, lever, and pencil are
replaced by a" diaphragm and mirror. The movement of the
diaphragm deflects the mirror, and a spot of light is thus
thrown on a moving screen. With a fast-running engine, the
spot of light traces what is apparently a continuous line, so
that the shape of the diagram is seen while the engine is
running. To obtain a record of the diagram, photography is
resorted to.

Figs. 47 and 48 show two indicator diagrams taken respectively
from the H.P. and L.P. cylinders of a 1,200 I.H.P. compound
condensing Corliss engine supplied for driving a mill. At the
time the diagrams were taken the engine was called upon to
give between f and J of its full power only, but as the dia-
grams are reproduced from those of large size and are taken
from a good engine, they will serve to show the use of such
diagrams, and how the power of an engine can be worked out
from them.

The explanation of the diagrams is briefly thus — The upper
line represents the pressure of the steam in the cylinder on the
outward stroke, the lower line the pressure on the return stroke.
When steam is first admitted to the cylinder the indicator pencil
rushes up to A; this initial pressure, in the diagram before us,
is well maintained until the point of cut-off, when the steam
begins to expand and the pressure falls. Point B shows approxi-
mately where the exhaust port begins to open ; it is fairly wide
open at C, and continues to remain open until D is reached
when the exhaust port closes, the steam remaining in the

124

MECHANICAL BNGINEBBING FOR BEGINNERS.

oylinder begins to be oompressed, the pressure rising to E. At
this point the steam port again opens, and the pencil rushes up
to A.

Fig. 47. — Indicator diagram.

The card taken from the other end of the cylinder was
practically identical, but is not shown in order to avoid
confusion.

Fig. 48. — Indicator diagram.

THB STEAM BNGINE. 125

The diagram shown by Fig. 47 was taken with a 40-lb spring,
so that every inch of height represented 40 lbs. pressure on the
piston. The diagram has been reduced in reproduction, but
a scale is given at the side so that the reader can measure
it for himself. To obtain the mean pressure shown by the
diagram, an engine maker would use an instrument called a
planimeter. By means of this instrument the area of any
irregular space can be measured by merely running the pointer
round the boundary line, when the area can be read off on
the index; if, knowing the area, we divide it by the length
of the diagram we get the mean height. However, as a plani-
meter is not possessed by eveiy one, the next best way to find
the mean pressure shown by the diagram is to divide it by 10
vertical lines, or ordinates, equally spaced. Then read off the
height (or pressure in pounds) of each by the scale, add them all
together, and divide by the number of lines. This will give the
average or mean pressure.

The particulars of the engine from which the diagrams were
taken are given below. The pressure shown by the boiler gauge
was 112 lbs. above atmosphere or 126-7 absolute.

Stroke, 6 feet.

Revolutions, 60 per minute.

Diameter of H. P. piston, .... 30 inches.
Area of H.P. piston, after deducting area of

6-inch piston-rod and tail-rod, . . 678*5 „

Diameter of L. P. piston, . . . . 56 ,,
Area of L.P. piston after deducting area of

piston-rod and tail-rod, . • . 2,434*7 ,,

Ratio of H.P. to L. P. cylinder, , . . 1 to 3-59

The mean pressures shown by the vertiocd lines are : —

H.P. diagram. L.P. diagram.

76 U -6

89 16

68 15-75

44 13-25

30 9-5

22 7-5

15 6

12 5

8 4

6 2-5

370 91

If we divide the above totals by the number of lines — viz., 10 —
we get an average effective pressure of 37 lbs. per square inch on
the H.P. piston, and 9*1 lbs. on the L.P. piston, or an equivalent

126 MECHANICAL ENGINEERING FOR BEGINNERS.

pressure of 19*4* lbs. on the L.P. piston. We do not need to
take any account of the back pressure against the pistons, as we
have measured the height o£ the vertical lines not from the
atmospheric line, or from the zero line, but from the line
forming the bottom of the diagrams, which shows the pressure
at which the exhaust steam leaves the H.P. cylinder to enter
the receiver, and the pressure at which the exhaust steam leaves
the L.P. cylinder to enter the condenser. Having obtained the
mean pressure from the diagrams, we can now calculate the
horse-power by the formula already given; the figures are —

2 X 5 X 60 X 2,434-7 x 19-4 o^^ t tt -o
3p00 ®^^ ^•^•^-

so that the engine was indicating 860 I.H.P. when the cards
were taken. The H.P. diagram enables us to see what drop in
pressure there was between the boiler and the cylinder. The
boiler pressure was 112 lbs. above atmosphere, and the card or
diagram shows that the highest pressure in the cylinder was
103 lbs. above atmosphere, or a drop of 9 lbs. The lower line
of the diagram shows the pressure at which the steam leaves the
H.P. cylinder; this pressure varies from 7 lbs. to 12 lbs. above
atmosphere, say 9 J lbs. average pressure; the top line of the
L.P. diagram shows that the highest pressure of the steam
received in the L.P. cylinder was 7 5 lbs. The drop, which is
not excessive, is caused, by the steam having to pass through the
exhaust port and receiver, and enter through the steam port of
the L.P. cylinder.

The consumption of steam in the engine was only 14*51 lbs.
per I. H.P. per hour ; an extremely good result for a compound
engine working at less than f load. It was not possible to
measure the actual load on the engine, so the mechanical
efficiency or consumption per brake horse-power is not known
as well as per indicated horse-power.

When it is not possible to measure the actual load, the ap-
proximate efficiency can be ascertained by running the engine
light and taking diagrams ; these will show the amount of steam
required to turn the engine round. This rough efficiency test
must be carried out before the driving ropes or belts are in
position. Assuming the mean pressure required when the
engine is fully loaded is 40 lbs., and the diagram shows a
mean pressure of 4 lbs. when running the engine light, the
engine efficiency is 90 per cent. This method is, however, not

* The L.P. piston is 3*59 times greater than the H.P. piston; a mean
pressure of 37 lbs. on the H.P. piston is therefore equivalent to, say, 10*3
lbs. on the L.P. piston— 10*3 + 91 - 19*4.

THE STEAM ENGINE. 127

very reliable, as, in the first place, diagrams showing such a
small mean pressure divided over the two cylinders are not
very accurate, and, in the second place, the friction of the
bearings and guides is greater at full than at light loads. It
has been found that with high-speed engines constructed on
interchangeable lines, and with the most accurate workmanship,
the mean pressure required to run them light is considerably
higher, and the consumption of steam greater, when first started
than it is after the engine has had a few days' running; by
which time the working parts will have acquired a fair working
face.

When indicating a Willans' single-acting engine, an indicator
card is always taken from the receiver beneath the H.P. cylinder
(also beneath the intermediate cylinder, if there is one), and the
mean pressure shown by such card is added to the mean pressure
acting upon the upper side of the piston. The reason for this
is as follows : — ^The lower line of the H.P. diagram shows the
pressure of steam above the piston while the cylinder is ex-
hausting to the receiver and the piston is on its up stroke ; as
soon as the piston reaches the top of its stroke tlie exhaust port
closes. On the down stroke the diagram shows the pressure of
steam driving the piston down, but it does not show what is
going on beneath the piston ; as a fact, the pressure beneath the
piston — i.e., in the receiver — ^begins to fall as soon as the L.P.
cylinder commences to draw steam from it. This fall of pressure
is shown by the receiver diagram, and is added to the pressure
acting above the piston.

129

CHAPTER VII.

THE STEAM ENGINE.

(Part II.)

Simple Flywheel Calculations. — In addition to being able
to work out the horse-power of an engine, it is essential that
a young engineer should be able to make simple calculations
required in connection with the flywheel, so that, for instance,
he may be able to compare the value of two flywheels differing
in size or weight, or to ascertain what effect the addition of a
few inches to the rim will have upon the stored energy, and
upon the tension in the rim due to centrifugal force.
The energy stored in a flywheel is expressed thus —

" 64-33 '

where W = weight of rim in lbs.

V == velocity in f6et per second at the centre of the mass
of the rim, or, as scientifically expressed, at the
radius of gyration

Example. — What energy is stored in the flywheel of the engine shown
by Fig. 35 ? We must first find the weight of the rim in pounds. There
are two ways of doing this — one is to t^e from a table of areas the area
of a circle corresponding with the outside diameter of the wheel, then
take the area of a circle corresponding with the inside of the rim, and
subtract one from the other. The result will be the superficial area of the
rim. In the case in point the diameter of the wheel is 3 > inches and the
area 1,017 inches ; the diameter inside the rim is 26 inches and the area
530 inches ; subtracting one from the other we have 487 inches ; the rim
is 5 inches thick, so we multiply 487 inches by 5 inches, and the result is
2,43) cubic inches of iron. We read in Chapter i. that 1 cubic inch of
cast iron weighs '26 lb., so that if we multiplv 2,435 by *26 we have the
weight of the rim in pounds— viz., 633 lbs. The other way is to find the
circumference of a circle running through the centre of the rim, — 97*4,
and to multiply this by the sectional area of the rim— 25 inches —
the result is the same — viz., 2,435 cubic inches. Having got the weight,
we must find out what V is. The engine runs at 3C0 revolutions per
minute, or 5 revolutions per second ; if, then, we take the circumference
in feet of the wheel at the centre of the rim — viz. , at the point indicated

130 MECHAKIGAL EKGINEERING FOB BEGINNERS.

by a cross* in Fig. 35 — and multiply it by the number of revolutions per
second, it will give us the velocity at which the rotating mass is moving.
The diameter of the wheel at the centre of the rim is 2 feet 7 inches, or
2*58 feet, and the circumference of a circle 2*58 diameter is 8*1 feet; if
we now multiply the circumference by the number of revolutions per
second, we get the answer 40*5 feet per second. By the formula we are
required to square this ; the square of 40*5 is 1,640. The calculation now

is oA.oo — = 16,137. The stored energy is, therefore, 16,137 foot-lbs.

If the student will work out the energy of a flywheel having a rim of
the same section — viz. , 5 inches — but the wheel to be 6 inches larger in
diameter, he will find that the stored energy has gone up greatly —viz. , to
27,000 foot-lbs.

The diameter of a flywheel is governed by the peripheral
speed at which it is safe to run. It is usually considered that
a cast-iron flywheel should not run at a peripheral speed greater
than 6,000 feet per minute ; at this speed the bursting stress in
the rim due to centrifugal force is only about 972 lbs. per square
inch, and even if the engine is called upon to run at 10 per cent,
above its normal speed, the bursting stress will only be about
1,176 lbs. per square inch. There is, of course, the possibility
of the engine racing to be taken into account, but even should it
race to the extent of 50 per cent., the bursting stress will be
only about 2,186 lbs. per square inch, or a little under 1 ton.
There is no doubt, therefore, that a peripheral speed of 6,000 feet
per minute, or 100 feet per second, is a very safe one for a fly-
wheel having a solid rim. If the flywheel is of such a size that
the rim has to be in two or more pieces, then the safety, or

* To obtain this point accurately the following formula should be used : —

A^ + B^

where A = outside diameter.
B = inside diameter.

This formula, which may perhaps look difficult to the beginner, is really
very simple. It shows that we square the outside diameter of the fly-
wheel, then squat e the inside diameter, add the two together, divide the
product by two, and then find its square root from a table of squares. The
calculation is as follows :—

/S&^ + 262 /l

296 + 676

2 ' "' ^ ""•

A table of squares and square roots will show that the square root of
986 is 31*4, so that we should take the circumference of a circle 31*4
diameter instead of one of 31 inches diameter obtained by measuring to
the actual centre of the rim.

THE STEAM ENGINE. 131

otherwise, depends on the method of jointing, and a speed of
4,000 feet per minute is probably sufficiently high. With fly-
wheels of very large size, say, 20 feet diameter and over, it
is not usual to run at a peripheral speed much higher than
3,500 feet per minute.

Centrifugal force is a convenient expression, but what it really
represents is covered by Newton's first law of motion. The law
reads thus : — " Every body continues in a state of rest or of
wmform motion in a straight line except so far as it is compelled
by force to change that state." Now, every particle in the rim
of a flywheel which has been set in motion tends to continue in
a straight line, and unless force is employed to make the particle
travel in a circle, the particle will continue to move in a straight
line, which forms a tangent to the circle. The force which has
to be employed to make the particle travel in a curved path is
the equivalent of the centrifugal force exerted.

To find the centrifugal force of a revolving mass the following
formula is used : —

w X yg

- 32-1 X R'

where W = weight in pounds.

V = velocity in feet per second;
R = radius in feet.

The following will, however, be found a more convenient
formula : —

C = WxN2xRx 1-226;

where W = weight in pounds.

N = number of revolutions per second.
R = radius in feet.

Example. — What is the centrifugal force of a piece of iron 1 inch
square, rotating at 960 revs, per minute at a radius of 1 foot ? The weight
of 1 cubic inch of cast iron is '26 lb., and 960 revs, per minute =* 16 revs,
per second ; the square of 16 is 256. The calculation is, therefore,
-26 X 256 X 1 X 1-226. The answer is 81*6. The force required to make
a piece of iron 1 inch square travel in a circle of 2 feet at the speed
mentioned, instead of going off at a tangent, is 81 '6 lbs.

Stress set up in the Rim of a Flywheel, — The formula given
above enables one to find the centrifugal force of any portion of
the rim of a flywheel, but to find the stress set up in the rim
and tending to burst it as the result of centrifugal force of all
its particles, it is necessary to find the centrSugal force of
1 cubic inch in the manner given above, then to multiply it

132 MECHANICAL SNGIKEERING FOR BEGINNERS.

by the diameter of the wheel, and divide the result by 2, or
expressed as a formula —

where B = bursting stress per square inch of rim.

C = centrifugal force of 1 cubic inch in pounds.
D ss diameter of wheel in inches.

The centrifugal force is multiplied by the diameter, and not by
the circumference of the wheel, for the same reason that the
stress upon a cylinder wall under internal pressure is found by
multiplying the diameter by the pressure. The reason is that
the pressure on the circumference is not all equally effective,
and the total effective pressure is equivalent to the pressure
multiplied by the diameter; and, as in a cylinder the total
pressure is borne by the two sides of a cylinder, so in a flywheel
the total bursting pressure is borne by the two sides of the rim ;
hence, C multiplied by D is divided by 2.

The formula gives the stress on each square inch of the rim
due to centrifugal force, and it is immaterial what the width
may be. It is best to work out the stress in a ring equal to the
outside diameter of the wheel and of 1 square inch section.
The stress on the inner portion of the rim, assuming the latter
is greater than 1 inch square, will, of course, be less than in the
outer portion ; but if a crack develops in the outer portion, the
inner portion will give way also.

Example. — What is the stress in the rim of a small wheel, 2 feet
diameter, running at 960 revs, per minute? From the example worked
out above we have seen that the centrifugal force of 1 inch of the rim is
81*6 lbs.; we therefore multiply this by the diameter — viz., 24 inches —
and divide by 2. The answer is 979 lbs. per square inch.

A very simple formula for finding the stress per square inch
in the rim of a flywheel is given by Professor Unwin in his
book on machine design. It is

where V = velocity in feet per second.

The figures 3-36 represent the weight of 1 foot of wrought
iron of 1 inch area. To make the formula applicable to cast-
iron flywheels, 3*12 should be used instead of 3-36.

An interesting feature about the centrifugal force acting on
flywheels is this : — If we have a 20-feet wheel running at 95*5
revs., a 5-feet wheel running at 382 revs., and a 1-foot wheel

THE STEAM ENGINE, 133

running at 1,910 revs., the peripheral speed is the same in each
case, but 1 cubic inch of cast iron placed on the rim of the large
wheel exerts a centrifugal force of 8*1 lbs. only. One cubic inch
of iron placed on the rim of the 5-feet wheel exerts a force of
32 lbs., while a similar piece of iron placed on the 1-foot wheel
exerts a force of 162 lbs. The explanation is simple. It is
this : — The path pursued by the piece of iron on the rim of the
large wheel approaches more nearly to a straight line than is
the case of the piece on the small wheel, and a smaller force is
required to make it travel in the necessary curve. In the small
wheel the departure from the straight line in a journey of 1 foot
is much greater, hence greater force is required to hold it in.

The size of the wheel does not, however, affect the bursting
stress per square inch of section of the rim, as, although in the
20-feet wheel the centrifugal force of 1 inch of metal is only
about 8-1 lbs., there is the equivalent of 240 of such pieces to
be reckoned with, while in the 1-foot wheel, although the force
is 162 lbs., there are only 12 such pieces to be allowed for.

The above formulae for finding the stress in the rim of a
flywheel assume that the tension is, all taken by the rim, the
holding-in power of the arms being ignored. In flywheels of the
disc pattern, as used on high-speed engines, the holding-in power
of the disc is very considerable. If we take the case of a disc
flywheel having a rim 6 inches wide by 6 inches deep, the disc
1 inch thick, the radius to the centre of the rim 1*5 feet, and
the number of revolutions 660 per minute, we shall find that,
although the speed is 6,200 feet per minute, the tension on
the disc (due to the centrifugal force of a section of the rim
6 inches x 6 inches x 1 inch) is only 2,081 lbs. per square
inch, so that even if the rim had a succession of saw cuts in it
6 inches deep, the disc would hold the pieces of the rim in place.

The author knew of a case where a 600 I.H.P. high-speed
engine got out of hand while being run for the purpose of
setting the governors ; the engine finally attained such a
terrific speed that it was completely wrecked, the only part
remaining undamaged being the solid disc wheel.

Stored Energy in Flywheels,— lX> may be asked what amount
of stored energy should a flywheel possess? The answer
depends upon the type of engine to which the wheel is to
be fitted, the work the engine has to do, and the regularity
of turning that is required. For instance, a double^ting engine
having three cranks placed at 120" apart will not require such a
heavy flywheel as an engine with two cranks at 180' apart. An
engine which has to drive a large circular saw and a dynamo
will require a much heavier wheel than one driving several

134 MECHANICAL ENGINEERING FOR BEGINNERS.

small lathes and a dynamo, for, in the case of the saw, the whole
load comes off and on suddenly. Again, an engine which is
required to drive a three-phase alternator in parallel with
others will need a heavier flywheel than a similar engine to
be used for driving the tools in an engineer's shop. To go
deeply into the question of flywheel weights and fluctuations
of speed, taking into consideration the varying pressures on the
crank-pin due to steam pressure, inertia of moving parts, &c., is
beyond the scope of this book, and unless the student is encraged
with a firm of engine builders, he need hardly concern himself
with such calculations. Even in engine builders' works the
amount of stored energy of the flywheel is usually settled
upon as the result of previous experience rather than as the
result of calculation. For instance, it was found that with a
certain type of single-acting three-crank engine, a flywheel
having stored energy of 3,000 to 4,000 ft.-lb8. per I.H.P. was
sufficient to meet the most onerous conditions, and a flywheel
having this amount of energy was accordingly provided when
the price obtained admitted of such a heavy flywheel.

In the huge flywheel, having a wrougnt-iron rim, fitted to
the earlier 5,000 H.P. turbines at the Niagara Falls, the stored
energy works out at about 22,000 lbs. per H.P., but the condi-
tions necessitating a flywheel having such an enormous amount
of stored energy are quite unusual.

Gfroverning Steam Engines. — The speed of a steam engine
is controlled by the governor and flywheel ; the former controls
the normal speed (sometimes called steady speed), while the latter
prevents any sudden variations of speed, and gives the governor
time to act. The governor may regulate the speed either by
throttling the steam before it enters the valve chest, or by
making the cut-off take place earlier or later during the stroke.

In the ordinary throttle governor the position of the balls
determines the amount of opening of the throttle valve, and
centrifugal force determines the position of the balls. The
valve is set so that when the engine is running at full speed th(B
passage of steam is unobstructed, or obstructed only so far as to
give a little higher pressure on the boiler side. If the speed of
the engine increases beyond the full speed for which it was
designed, the balls open outwards and the throttle valve partially
closes ; if the speed of the engine falls, the balls close either
by their own weight, or assisted by springs, and the throttle
valve .opens. When the engine is at rest the throttle valve is
wide open.

In a governor which controls the speed of the engine by
varying the expansion, as in a Corliss engine, the motion of the

THE STEAM ENGINE. 135

balls causes the catch plates to release the steam admission valve
at an earlier or later portion of the stroke.

It should be clearly understood that a governor, however
sensitive, cannot govern an engine to one absolutely uniform
speed, as it is only by a variation of the speed that the centri-
fugal governor comes into play. A good governor should,
however, be able to control the steady speed of the engine to
within 3 per cent, under all changes of load, so that if the
governor is set to give a speed of 100 revs, per minute under
full load the engine should never run faster than 103 revs, even
without any load. If the whole load is thrown off suddenly,
the speed may rise momentarily beyond the 103 revs., but the
governor will bring it back in a few seconds. The extent of the
momentary variation of speed depends more upon the flywheel
than upon the governor; therefore the flywheel should be
sufliciently heavy to keep the momentary variation of speed
down to 5 per cent, until the governor is able to produce its
effect. When a complaint is made that an engine will not
govern under sudden changes of load the fault may be looked
for rather in the flywheel than in the governor.

A typical specification as to governing is the following: —
"The governor to be of an approved type capable of easy
adjustment by hand while the engine is running. The
governor to control the speed of the engine within 3 per
cent, between full load and no load, and with a temporary
variation of not more than 5 per cent, under any variation
of load."

If an attempt is made to govern an engine much closer than
is indicated by the above limits, or to make the governor
"isochronous," there is a tendency for the governor to hunt.
What is called hunting is this ; when the governor is fitted with
too sensitive springs and an increase of speed takes place, the
throttle valve closes suddenly and thrott'es the steam to such an
extent that the speed of the engine falls below the normal ; the
throttle valve then opens suddenly and allows rather too much
steam to pass, when the speed rises above the normal; the
governor again closes suddenly, and so the hunting action goes
on. This hunting may give rise to very irregular running, and
it is preferable to have a governor that will control the speed of
the engine within reasonable limits, and be "stable," than to
have one fitted with such light springs that it will hunt on
the least provocation.

The balls of a governor should be fairly heavy, and con-
trolled by strong springs if the spindle is horizontal, or by a
weight if vertical. If the balls are light and the springs weak

136 MECHANICAL ENGINEERING FOR BEGINNERS.

any friction of the governor mechanism may seriously interfere
with the governing.

It is more difficult to govern within close limits a compound
or triple-expansion engine than a simple engine unless the cut-ofif
can be varied in each cylinder, as there is a certain amount of
steam in the receivers which must be got rid of before the full
effect of throttling or cutting off the admission in the H.P.
cylinder is felt.

Some engines are fitted with an emergency governor. This
governor does not come into action unless tne main governor
fails to hold the engine, and the speed assumes dangerous pro-
portions. The governor of an engine should not be driven by a
belt, as this may slip or break. In high-speed engines it is a
common practice to fit the governor to one end of the crank
shaft, and thus do away with any gearing ; in cases where this
is not practicable the governor should be driven by gearing.

BelatiTe Eoonomy of Governing by Throttling or by
Varying the Expansion. — It is generally believed that it is
more economical to govern an engine by altering the point of
cut-off than to govern it by throttling down the steam at light
l6ads. It is contended that if the boiler pressure is, let us say,
160 lbs., it is better to work with this pressure at all loads
rather than throttle down the steam to 60 or 70 lbs. at very
light loads. This is, in the main, correct, and with low admis-
sion pressure it is undoubtedly more economical to vary the
expansion whatever the load may be than to further throttle
down the steam ; but when high initial pressures have to be
dealt with the case is rather different, for if the expansion is
carried out to too great an extent the range of temperature in
the H.P. cylinder is very great, and the initial condensation
is excessive.

If the load on the engine is so light that the steam is expanded
until the mean pressure in the H.P. cylinder is about one-tenth
of the admission pressure, then it is more economical to throttle
down the steam and to expand less.

For any variations of load less than the above, the variable
expansion type of governor is more economical than a governor
of the throttling type.

In cases where an overload may come upon the engine, the
variable expansion type of governor is far preferable to the
throttling type. For instance, an engine is fitted with a throttle
governor, and the full load is obtained with, say, 160 lbs. pres-
sure and '5 cut-off; if a sudden overload should come upon the
engine it cannot respond, because the governor obviously cannot
raise the stedm pressure above that of the boiler, and the point

THE STEAM ENGINE.

137

of cut-off is fixed ; whereas if the engine is fitted with variable
expansion gear, as soon as the speed of the engine begins to fall
the governor makes the point of cut-off later, and so increased
power is obtained.

Proportion^ of, and Stresses in, Various Farts of a
Steam Engine.— The following data, which are based upon
present-day practice, may be useful : —

Ratio of Cylinders. — ^The ratios which the intermediate and
L.P, cylinders bear to the H.P. cylinder are usually as follows : —

Compound Non-
condensing Engines.

Compound
Condensing Engines.

Triple Condensing Engines.

H.P.

LP.

H.P.

L.P.

H.P.

LP.

L.P.

2-5

2-25

3-5

2-5

6-5

Area of Forts. — The area of the steam ports should be such
that the flow of steam through them does not exceed 7,000 feet
per minute. In the case of a locomotive running at 60 miles
per hour, the speed of the steam through the ports is sometimes
as high as 11,000 feet per minute, but at this speed the steam is
seriously throttled. The area of the exhaust ports of Corliss
engines is usually about IJ times that of the steam ports. In
the case of slide-valve engines the exhaust port is made from
1^ to 3 times the width of the steam ports. The exhaust port
of the L.P. cylinder which communicates with the condenser
should be made as large as the general design of the cylinder
will admit of.

Having decided upon the speed at which the steam is per-
mitted to pass through the ports, the area is found thus —

Area of port in inches =

where P = piston area in inches.

S = piston speed in feet per minute.
V = velocity in feet per minute at which steam is per-
mitted to pass through port.

Thickness of Cylinder Walls The thickness of a cylinder

wall is siich that the stress up(m it is about 1,500 lbs. per
square inch, with a minimum thickness of ^ inch. The thick-

OF THE

UNIVERSITY

138 MECHANICAL ENGINEERING FOR BEGINNERS.

ness (without reference to the minimum) can, therefore, be
found as follows : —

3,000 " '

where D = diameter of cylinder in inches.
P = highest admission pressure.
T = thickness of cylinder in inches.

The thickness of the H.P. and L.P. walls is usually the scune.

Piston-rod. — ^This must be of sufficient size to withstand the
alternate tension and compression due to the pressure on the
piston (and to its inertia) ; it must also be sufficient stiff to
resist any tendency to bend or buckle under its load. The stress
usually allowed is about 3,000 lbs. upon each square inch of the
full diameter of the rod ; the stress at the smallest part of the
rod — viz., at the bottom of the thread cut for the nut used to
hold the piston, the rod having previously been tapered down
for the piston — may be as much as 5,500 or 6,000 lbs. per square
inch. In locomotive practice the rod is usually enlarged where
it fits into the piston, and before the taper begins ; even then
stresses up to 7,500 lbs. per square inch are often met with.
The piston-rods of the H.P., I.P., and L.P. pistons are usually
of the same diameter, and it will be found that this is usually
from ^ to -^ the diameter of the L.P. piston. In large marine
engines, or in engines where all the parts are light, the rod is
often only -^ the diameter of the L.P. piston, but in the case
of Corliss engines and of compound locomotives the diameter
is usually about 4 that of the L.P. piston.

ConnectiDg-rods. — The usual practice as regards length of
connecting-rod is as follows : —

Marine engine, 3*5 to 5*0 times length (or radius) of crank,
stationary 14.75405.5

land engines, J " » »

Locomotives, 5*5 to 7*0* „ „ „

The sectional area of the connecting-rod should be equal in its
smallest part to that of the piston-rod, and the section should
increase gradually towards the crank-pin end until it is 1 J to 1 J
times the area of the piston-rod.

Eccentric-rods and Valve-rods. — ^The size of these depends
largely upon the character of the valve gear, and no general
rules can be given.

* In the case of a four-cylinder, ten-ooupled locomotive recently con-
structed by the Austro- Hungarian Railway, the connecting-rod was 9*4
times the length of the crank.

THE STEAM ENGINE. 139

Pressure on Journals, Crank-pins, and Grosshead-pins. — The
pressure which may safely be allowed depends to a great
extent upon the speed of the rubbing surfaces; thus a much
greater pressure is usually allowed upon the crosshead-pin than
upon the main bearings. The pressures usually allowed are as
follows : —

Main bearings, . 300 to 500 lbs* per square inch.
Crank-pins, . . 600 to 900 „ „

Crosshead-pins, . 1,200 to 1,500*,, „

The above pressures are reckoned on the projected area of the
bearing— t.e., the diameter multiplied by its length, and not half
the circumference multiplied by the length.

Diameter of Crank-shafts. — Crank-shafts of compound and
triple-expansion engines of the marine type are usually made
approximately one-fifth the diameter of the L.P. cylinder.
Where there are two L.P, cylinders to one H.P. or intermediate
cylinder, then the crank-shaft will be found to be approximately
one-fifth the diameter of a cylinder having an area equivalent
to the two L.P. cylinders. In the case of high-speed engines,
where the reversal of stress in the shaft is very frequent, the
crank-shaft is usually made about one-quarter the diameter of
the L.P. cylinder. This rule does not apply to the crank-shafts
of gas engines or to engines which have a heavy flywheel or rope
pulley placed between the .cranks, as is frequently the case with
horizontal land engines.

Pressure upon Slide Bars.— The pressure upon the slide bars
is found by multiplying the total pressure on the piston, by
the length of crank, and dividing the result by the length of
connecting-rod. There is usually no difficulty in providing slide
bars and slipper having ample surface, and the pressures upon
them are consequently light ; they vary from 50 lbs. per square
inch of surface in stationary engines to about 125 lbs. per square
inch in the case of marine engines and locomotives. It may be
stated here that friction theoretically is independent of the
extent of rubbing surface, and depends only upon the pressure
between the surfaces in contact. The coefficient of friction of
dry steel to steel is about '18, so that if we have a piece of steel
weighing 100 lbs. pressing on another piece, it will require about
18 lbs. to move it about. If, however, we can keep a film of oil
between the surfaces the friction is considerably reduced. The
best way to ensure a film of oil remaining between the surfaces
is to provide ample area, and consequently reduced pressure.

* Exceeded in locomotives.

140

MECHANICAL .ENGINEERINa FOR BEGINNERS.

Some experiments have been carried out at Cooper's Hill
Engineering College*^ in order to ascertain the coefficient of
friction between wrought iron and steel journals and bearings
of different alloys, the journal and its bearing being immersed
in a bath of oil. The following are approximately the results
obtained with a steel journal running in a phosphor-bronze
bearing : —

Coefficient 'of Friction.

Pressure on Bearing per
Square Inch.

When Peripheral Speed

of Journal = 400 Feet

per Minute.

When Peripheral Sjpeed
per Minute.

600 lbs.
600 ,.
400 „
300 „
200 ..
100 „

•0032
•0035
•0040
•0045
•0060
•0095

•0029
•0029
•0030
•0033
•0040
•0054

It is probably impossible to ensure such a good film of oil
remaining between sliding surfaces as between a shaft and
bearing running in oil, as the shaft draws in oil in the same
way that a pair of rolls draws in a sheet of metal. It is,
however, advisable to provide large rubbing surfaces, so that
the oil is not so easily squeezed out, and in this sense the
statement that friction is independent of surface has to be
qualified.

Piston Speeds. — The piston speeds usually found in actual
practice are approximately as follows : —

Corliss engines,
Marine engines.
Locomotives, .

. 500 feet per minute.
. 500 to 1,200 feet per minute.
. 1,120 feet per minute when running
at 60 miles per hour.

Engine Packings. — In the early days of the steam engine,
when steam pressures were low, it was customary to pack the
glands of the piston- and valve-rods with hemp soaked in tallow,
but with the advent of high pressures it was found that hemp
charred, owing to the heat. When this occurred, the engine
driver, in order to keep the gland tight, screwed down his gland
tighter and tighter; the result was frequently a scored rod.
Asbestos fibre packing was introduced about the year 1870, and

* See Engineering, vol. Ixxxii., p. 695.

THE STEAM ENGINE.

141

was found far superior to hemp for high-pressure steam, and it
is very largely used at the present time. The method of packing
a gland by squeezing any form of fibrous material up against the
rod and walls of the gland is, however, open to objection. In
the first place, if the driver screws down his gland nuts too
hard, an excessive amount of friction on the rod is caused; it is
possible to pull up an engine of about 50 H.P. by tightening
the glands excessively. Secondly, the pressure on the rod is tl^
same on both the steam and exhaust strokes. ' Thirdly, any slight
lateral movement of the rod tends to cause the packing to leak.

49. — Metallic packing.

To overcome these defects various forms of metallic packing,
consisting chiefly of soft white anti-friction metal rings or blocks,
have been designed and patented. The best form of packing of
which the author has any knowledge is that shown by Fig. 49*
This packing was designed by a Scotch engineer, Mr. Monroe,
patented in the United States, and is made and sold in this
country under the slightly misleading name of the United
States Metallic Packing. Fig. 49 shows the packing as supplied

142 MECHANICAL ENGINEERING FOR BEGINNERS.

for very high pressures ; for low-pressure cylinders the left-hand
part of the packing only is required. The packing consists of
eight white metal blocks, arranged so that they break joint;
each block is pressed on to the rod by a light spring, so that the
• pressure on the rod is about IJ lbs. per square inch only; on
the steam stroke, however, the pressure of the steam is added to
that of the springs, and a tight gland is the result. On the
exhaust stroke tibe pressure of ihe steam is removed, the
pressure of the springs being sufficient to ensure the gland
remaining tight under low pressure, or under atmospheric
pressure when the engine is condensing. The white metal
rings shown in the right-hand portion of the packing are
for the purpose of reducing the pressure of the steam, when it
is very high, before it reaches the packing blocks already
referred to. As the reader will see from the illustration, the
packing admits of lateral play of the rod without affecting the
steam tightness; also, that it is impossible for the driver to put
any undue pressure on the rod, as there are no gland nuts to
tighten.

The form of packing described is very largely used in marine
as well as in land work ; the only objection to it is that the first
cost is greater than that of a gland arranged to receive asbestos
packing, and that occasionally the springs break. Against
these objections it is claimed that, by reducing the friction on
the rod, the mechanical efficiency of the engine is increased by
2 per cent, or more, and that the cost of replacing a spring is
trifling.

Piston Rings. — In the v^y early days of the steam engine
the piston was packed by means of rope or "junk," fitted into a
recess, and held in position by a junk ring. The first to depart
from this plan was Mr. Bamsbottom, who used narrow piston
rings as shown by Fig. 52. These rings were turned slightly
larger than the cylinder, were cut across, and sprung inwards ;
the spring in the material was sufficient to cause the rings to
press tightly against the walls of the cylinder. Rings of this
description are still largely used in locomotives and in fast-
running petrol engines. There is nothing in them to go wrong,
and they do not require a junk ring.

The rings are generally cut through at an angle, as shown by
Fig. 50, so that there is not a direct path for the steam to blow
through. When the rings are put into the piston the openings
are spaced equally round the piston, but it is found that the
rings have a tendency to work round into the position shown by
Fig. 50. It is said that this tendency can be frustrated by
cutting through the central spring at an angle inclined in the

THE STEAM ENGINE.

143

Opposite direction to the angle of the others, as shown by Fig.
51. The objection to the Bamsbottom form of ring is that it

Fig. 50.

Fig. 53.

*Tmi\

Fig. 56.

Fig. 61.

Fig. 54.

Piston rings.

Fig. 62.

Fig. 56.

Fig. 57.

cannot be replaced without drawing the piston, unless the latter
is specially constructed, as shown by Fig. 38, and that, being

144 MECHANICAL ENGINEERING FOR BEGINNERS.

originally made of slightly larger diameter than the cylinder, it
does not press equally all round the latter when sprung in.

A simple form of piston packing, and one which has proved
efficient in sizes up to 4^ inches diameter, is that used in the
Willans engine; a sectional view of the packing is shown by
Fig. 55. The packing consists of two rings, A, A, turned to the
exact diameter of the cylinder, so that they bear equally all the
way round, and one cast-iron inner spring, B; this spring is
turned slightly larger than the bore of the cylinder, and is
thicker at one portion than at another, as shown by Fig. 53 ; it
is then cut, sprung inwards and placed inside the two thinner
and concentric rings, A. The tendency of the spring B to
resume its original form makes the rings A press evenly against
the cylinder walls. The piston rings and spring require to be
an extremely good fit between the junk ring and piston flange C,
otherwise there is play, and the springs hammer themselves into
the junk ring and piston flange. The effect was tried of making
the junk ring of thin steel plate, the idea being that the steam
would spring the steel plate on to the rings and spring, and thus
prevent any play, but the experiment was not a success.

In the Mudd form of packing, shown by Figs. 56 and 57, any
play of the rings between the junk ring and piston flange is

Prevented by means of a number of spiral springs as shown,
'he rings are kept up against the cylinder walls by means of
springs placed between the ends of the former, as shown at S, S
in the small scale plan of the ring, Fig. 6. This form of packing
is used largely in marine work, and has proved satisfactory.
There are a very large number of patent piston rings and springs
besides those mentioned, but the examples chosen will seive, as
well as any, to make the principles clear.

Efficienoy of Steam Engines. — ^The term efficiency is often
used in widely different senses. For instance, when a man says
that his engine, although rather old-fashioned, is still very
efficient, he probably means that the engine does not break
down or cause trouble, and that he, the owner, is ignorant of,
and indifferent to, the consumption of steam. Another man, in
stating that his new engine is extremely efficient, probably refers
to the consumption of steam per indicated horse-power ; while a
third, in endeavouring to sell a somewhat uneconomical engine,
may lay stress upon its high mechanical efficiency. The mechani-
cal efficiency of an engine, as already explained, depends solely
upon the amount of internal friction.

The real efficiency, from the purchaser's point of view, is the
consumption of steam at a given pressure (and temperature) per
brake or effective horse-power.

THE STEAM ENGINE. 145

The true thermal efficiency of an enirfne is the ratio the number
of thermal units represented by the actual horse-power developed^
bears to the number of thermal units put into the steam con-
sumed by the engine. The efficiency of an engine reckoned in
this way is so low (about • 1 57 in the case of a compound con-
densing engine, using 15 lbs. of steam per I.H.P. per hour,
working with 150 lbs. boiler pressure) that it is seldom used in
commerce, and is chiefly useful in comparing the performance of
a steam engine with that of, say a gas engine.

Then, again, the thermal efficiency of a steam engine may be
compared with that of a perfect steam engine working within
the given limits of admission and exhaust temperatures. Such
a standard has been recommended by the Institution of Civil
Engineers (see vol. cxxxiv., p. 294 of these Proceedings)* The
standard chosen is that laid down by Clausius and Rankine, and
not that of the Carnot heat cycle.

Before concluding the chapter on steam engines, a few words
must be said on the subject of the Zeuner diagram. This
diagram enables the designer to see at what part of the stroke
the cut-off takes place with a given amount of lap and lead;
also the points at which the port opens to exhaust, and where
compression begins.

Perhaps a little incident which actually occurred may show
the beginner how the ability to construct such a diagram helped
at least one young draughtsman a step forward. This young
fellow, who wished to gain further experience, accepted a berth
as draughtsman with a small firm of mechanical engineers on the
coast. The firm had under construction a small marine engine
of a size not previously made, and when it came to drawing
out the slide-valve, the principal, who was an extremely good
practical engineer, but who had not much theoretical knowledge,
said to the draughtsman — " You had better make the lap threo-
quarters of an inch; I think that will be about right.'* The
reply was — " Very well, sir, if you like I will set out a Zeuner
diagram, so that we may see what the effect will be." The
principal replied — "Oh, I think three-quarters of an inch will
be near enough." However, after office hours, the draughtsman,
for his own satisfaction, set out the Zeuner diagram, and the
following morning showed it to his principal. The principal,
whose guess as to the right amount of lap had been a good one,

* These volumes can be seen by anyone in London free of charge at the
library of the Patent Office in Southampton Buildings, Chancery Lane.
This ubrary contains the past volumes of the Engineer and Engineering ,
and other technical papers ; aUo a large collection of valuable text-books'
dealing with various subjects.

146

MECHANICAL EN6INEEBING FOB BEGINNERS.

seemed very pleased to see set out so clearly the exact points
where cut-ofif took place and where compression began, and at
the end of the week the draughtsman was gratified to find that

Fig. 58.

---_,

OFF

^^^^^^

f
1

_ _ _ — - ■■

• " 1

Exhaust
C/oses

Fig. 59. — Zeuner valve diagram.

his salary had been increased. The principal, when thanked,
merely said — "You are worth more than you are getting."
Had the principal's guess turned out a bad one, it is just possible

THE STEAM ENGINE. 147

that he might not have been so pleased with the Zeuner diagram,
but it would have been accurate just the same.

The diagram (Fig. 58) is constructed as follows : — Draw AB
parallel to the line of the stroke. With a radius = J the valve
travel draw from the centre E the circle BOAK. Mark off
EF = the lap, and FG = the lead. Draw the perpendicular
GH. Join EH. Then V = angular advance. On HK draw
the valve circles as shown. From E draw lap circles with radii
EF = outside lap and ER = inside lap ; then —

EB ^ position of crank at beginning of stroke.
EM = „ „ at cut-off.

EN = „ „ when exhaust opens.

EO = „ „ »> » closes.

EP = „ „ „ steam port opens.

Having found the position of the crank at the points of
admission, cut-off, compression, <&c., one has merely to draw the
connecting-rod and cylinder, as shown by Fig. 59, to obtain
the corresponding position of the piston. In the illustration the
connecting-rod has been assumed to be five times the length of
the crank; EM is assumed to be the crank; therefore the
connecting-rod is drawn five times the length of EM. One end
of the cylinder will then be a connecting-rod's length from A
and the other end a rod's length from B. By drawing the
connecting-rod and piston, any position of the crank, found by
Fig. 68, will give the corresponding position of the piston in
the cylinder.

149

CHAPTER VIII.
POWER TRANSMISSION.

Belts, Hopes, and Gearing. — ^When an engine is not coupled
directly to the machine it is required to drive, such as a dynamo,
or, let us say, to a propeller shaft on board ship, and it is
necessary to transmit the power to a machine some distance
away, such transmission is usually eflfected by means of belts or
ropes, pulleys, and steel shafting. For powers up to 100 H.P. flat
leather belte are generally used ; such belts can transmit up to
200 or 250 H.P., or more, but ropes are usually preferred when
powers greater than 100 H.P. have to be dealt with.

The amount of power a belt is able to transmit depends upon
the width of the belt, the speed at which it runs, upon the
strength of the belt and its fastenings, and the extent to which
the belt laps round and grips the pulley.

In a well-arranged horizontal belt drive the driving side is
underneath, the slack side being uppermost, so that the sag of
the slack side causes the belt to wrap itself more completely
round the pulley than would be the case if the driving side were
on the top and the slack side underneath. In the majority of
cases of belt transmission the belts are placed at an angle.
A very steep drive — i.e., one in which the belt is nearly vertical
— should be avoided if possible, as the more nearly the drive
approaches the vertical the greater the tendency of the belt to
slip. To avoid such slip the belt requires to Ibe very tightly
laced, and this causes i^ndue friction on the bearings of the
shaft, and consequent loss of power.

The power which can be transmitted by leather belts can be
found approximately by the following formula: — *

TWV

•^^^ ^ - HP •

33,000 •'

* To be Btriotly accurate the formula should take into account the
exact extent of the circumference of the pulley embraced by the belt, but
in ordinary practice the width of a belt is never cut down to such a fine

Soint that calculations going into these minute points need be made. A
raughtsman who would spend a morning in making calculations of this
nature in connection with the width of a pulley, would be of little use to
his employer. The efficiency of a draughtsman is usually reckoned by the
following formula: —

where E = efficiency of draughtsman.
E - ^ ^ -^ . ^ = work turned out.

T ' -A. = accuracy, sufficient for all practical purposes.

T = time occupied.

150 MECHANICAL EKGINEERING FOR BEGINNERS.

where T = working tension in pounds per inch of width.
W = width of belt in inches.
V = velocity in feet per minute.

The safe working tension of leather belts depends upon the
thickness, and as the thickness of hide does not vary much,
additional thickness is given by placing two or more hides
together. A single belt is formed out of a thick single hide and
is about ^ inch thick ; a double belt consists of two thicknesses
of hide and may vary from J to f inch thick. For the purpose
of the above formula the safe working tension of leather belts
may be taken as follows : —

Single belt j% inch thick, T = 55 lbs.
Double belt | inch thick, T = 80 lbs.

Let us work out an example.

Example, — What horse-power can be transmitted from a 3-foot pulley
running at 200 revolutions per minute, the face of the pulley being
10 inches wide and the belt used, say, 9 inches wide? To find the velocity
we multiply the circumference of the pulley, which is 9*42 feet, by the
number of revolutions per minute, 9*42 x 200 = 1,884. We will assume
that a single belt is used, the safe working tension of which we read above

is 66 lbs. Thecalculation then is55jL?-^i-L?§f==: 28*2 H.P. The answer

33,000
is 28*2 horse-power.

If a double oelt had been used, or if the velocity had been greater, the
belt would have been capable of transmitting more horse-power.

If we wish to find the width of belt required to transmit a
certain horse-power the formula transposed is

^ _ H.P. X 33,000
TV

In cases where large powers have to be transmitted at low
speeds, belts formed of leather links are sometimes used ; these
belts are more flexible than very thick solid belts ; also as air
can get away through the links they grip a wide pulley better
than a solid belt, and are considered capable of transmitting
about 30 per cent, more power. Centrifugal force, however,
renders these belts unsuitable for high speeds, on account of
their great weight.

Leather belts will run satisfactorily up to a speed of about
3,000 feet per minute ; beyond this speed centrifugal force
interferes somewhat with the gripping action. In cases where a
belt will not transmit the required power, owing either to the

POWER TRANSMISSION. 151

result of centrifugal force, or to insufficient width, the required
power can usually be obtained by using two belts, one over the
other ; this is sometimes called compounding. As the outer belt
travels a little faster than the inner one and is usually narrower^
the centrifugal forces do not act in the •same way on the two
belts and a better gripping action results. The power trans-
mitted by two belts placed one over the other is greater than
would be the case if the two belts were placed side by side on a
very wide pulley, as apart from the centrifugal action one
cannot be sure that the tension is the same in the two belts
when so placed.

Leather belts when compounded have run satisfactorily up to
speeds of 8,000 feet per minute.

Canvas bejting is sometimes used for driving purposes ; it is
very flexible, but its life is not so long as that of a leather belt.
The ends of leather belts are fastened together by laces. Many
forms of metal fasteners are sold, but it is doubtful whether a
joint made by them lasts as long as a properly laced joint.

Pulleys for driving belts are made of cast iron, wrought iron^
and are sometimes built of wood. If the pulley is to be placied
at the end of an engine or other shaft it is made solid and keyed
on, but the majority of pulleys are now made in halves so that
they may readily be put on the shaft while the latter is in
position. Wrought-iron pulleys are slightly more expensive
than cast iron, but are usually preferred on account of their
lightness and strength. If the pulleys are to run at a high
speed they must be properly balanced — i.e,, no part of the rim
must be heavier than another unless exactly balanced by a
corresponding weight on the opposite side. If this balance is
neglected the pulley will run untruly, and may distort the shaft
or loosen the bearing.

The faces of pulleys for leather belts, with the exception of
fast and loose pulleys, are always "crowned" — i.e., slightly convex;
if they are not crowned the belt will run off. A leather belt
always tends to mount the highest part of a pulley face; hence
if the centre of the face is the highest part, the belt will run up
to the centre of the pulley and remain there. If the convexity
of a pulley is too great, the centre portion of the belt only grips
the surface and the belt fails to transmit the power it is capable
of. It has been found in such cases that by reducing the con-
vexity of the pulley greatly increased power is transmitted.
The rule frequently given in text-books that the convexity of
a pulley should be from J to J inch for every foot width of
pulley, if acted upon, would probably give rise to the trouble
referred to.

152 MECHANICAL ENGINEERING FOB BEGINNERS.

In the opinion of the late Mr. TuUis, who probably had more
experience in connection with belt driving than most men, a
convexity of ^ inch is sufficient for pulleys up to 6 inches wide,
and a convexity of ^ inch for very wide pulleys. If, however,
the shaft is verticil •and the pulley horizontal (an unusual
arrangement except in the case of some machine tools), the
convexity should be doubled.

By varying the size of the pulleys the relative speed of the
driving and driven shafts can be varied through a wide range,
but it is not good practice to have a greater difference than 4 to 1
between any two pulleys. That is to say, a 4-foot pulley should
not drive one smaller than 1 foot diameter, or greater than 16
feet diameter ; il^ this ratio is exceeded the extent of the circum-
ference of the small pulley embraced by the belt or rope is small.
When a large ratio must be given the pulleys should be placed
as far apart as possible.

In cases where the two pulleys cannot be placed a good
distance apart, and it is necessary for the belt to lap well round
the small pulley, thR belt is sometimes left rather slack, and a
third pulley is used to press the slack portion of the belt towards
the tight portion. This third pulley is called a Jockey pulley.
It is not often employed, as it is noisy, and tends to wear the
belt out quickly.

The speed at which a driven pulley of a given size will run, if
the diameter and speed of the driving pulley are known, is found
as follows: —

D X S

where D = diameter of the driving pulley.
S = speed of the driving pulley.
d = diameter of the driven pulley.
8 = speed of the driven pulley.
Exam/fiU. — Suppose we have a pulley, 3 feet in diameter, on a shaft
running at 200 revs, per minute, which drives by l)elt a pulley 2 feet in
diameter, at what speed will the latter run? By the above formula
3 X 200 = 600 4- 2 = 300. The answer is 300 revs, per minute

If we know the diameter and speed of the driving pulley, and
require to know what diameter the driven pulley must be in
order to make the latter run at a given speed, the formula is
merely transposed thus : —

D X S ^

Example. — Suppose the driving wheel is 3 feet in diameter, and runs at
200 revs, per minute, what diameter must the driven wheel be to give a
speed of 120 revs, per minute? The calculation is 3 x 200 ~- 120 = 5 feet.

POWER TRANSMISSION.

153

To ensure a long life for a belt, the pulleys should be of
adequate size. In cases where the pulley must be of small
diameter a thin belt should be used.

The following are suitable thicknesses of belts for small
pulleys : —

Pulley 4 inch or less diameter ; Belt J inch thick.
„ 41 to 8 inch „ „ ^ „

In all cases where a machine is driven by belt from the
main shafting, means must be provided for starting and stopping
it without arresting the progress of the main shafting. This is
eflfected by means of fast and loose pulleys ; the pulley on the
main shaft is a wide one, not crowned ; the machine is provided
with two pulleys side by side, one of which is free to revolve on
its spindle. When the machine is at rest the belt drives the
free pulley; when it is desired to start the machine the belt is
moved by a fork and lever, called " the striking gear," on to the
fixed pulley, and so the machine is driven.

In many cases, as in most lathes, it is usually more convenient
to place the fast and loose pulleys on a shaft overhead. The
countershaft, as it is called, has pulleys of different sizes corre-
sponding with similar pulleys on the lathe, called cone pulleys;
this arrangement permits of the speed of the lathe being varied :
thus, when the belt is on the large pulley of the countershaft, it
is on the small pulley of the lathe, and the latter runs fast.
When the belt is on the small pulley of the countershaft, and on
the large pulley of the lathe, the latter runs slowly. The fork
of the striking gear prevents the belt from running off the
uncrowned faces of the fast and loose pulleys.

Hope Driving. — The power transmitted by ropes may be
found from the following figures, which err, u at all, on the
safe side: —

One rope . .

2-6
3

4-5
6

inches in diameter

Will transmit-!

2
2-5

8
10

10
13

rhorse-power if hemp
\ ropes are used
f horse-power if cotton
\ ropes are used

for every 1,000 feet velocity per minute.

154 MECHANIOAL ENGINEERING FOB BEGINNERS.

Example, — Suppose we have a pulley, 38 inches in diameter, which has
grooves for ten IJ-inch ropes, the speed is 470 revs, per minute, what
power will it transmit? We must first fiud the velocity: the circum-
ference of a 38-inch pulley is 119*4 inches, or 9*95 feet; we multiply this
by the number of revolutions, and the result is 4,676 feet velocity per
minute. By the above rule one 1 J-inch cotton rope will transmit 5 H.P.
for every 1,000 feet velocity; therefore, ten ropes will transmit 5<) H.P. for
every 1,000 feet velocity. The velocity we have found is 4*67 thousand
feet per minute, therefore 4*67 x 50 = 233*5. The H.P. transmitted
is 233*5.

A rope pulley should be not less than 30 times the diameter
of the rope, if a cotton rope is used; or 40 times the diameter, if
a hemp rope is used. Thus, if we have a pulley 30 inches in
diameter we must not use cotton ropes larger than 1 inch in
diameter, or hemp ropes larger than f inches in diameter. A
small rope bends more easily than a large one ; therefore, as
a general rule, it is better to have a good many small ropes
than a few of large diameter.

Ropes may be run up to a speed of 7,000 feet per minute, but
a speed of between 4,500 and 5,000 feet per minute is considered
the best. Cotton ropes are more expensive than hemp ropes^
but as they last much longer, they are really cheaper in the long
rim. Makers of ropes usually speak of them by the circum
ference; thus a maker's 3-inch rope would be one of about
i inch diameter. Ropes should not touch the bottom of the
V-shaped groove in the pulleys, but should wedge themselves
against the sides. The best angle for the sides of the groove is
about 40" lyhen the diameter of the rope is over 1 inch, and an
angle of about 30** when the rope is less than 1 inch in diameter.

When the diameter of a rope pulley is spoken of, the effective
diameter — %,e., the diameter of the pulley where the ropes grip
the sides — ^is meant. The ends of ropes are spliced together;
the rope makers isend out men specially qualified for doing this
work.

It has been found in practice that in cases where the drive
is irregular, as with a gas engine, a steadier drive can be
obtained by placing the slack side of the rope underneath,
and the driving side uppermost, than by the opposite
arrangement.

A question which frequently occurs in laying out a rope drive
is : — What is the minimum distance at which the pulleys should
be placed apart 1 A rule given by Mr. Kenyon (an authority on
rope driving) is as follows: — Take the difference between the
diameter of the largest and of the smallest pulley and add it to
one and a-half times the diameter of the largest; the result
gives the distance between centres of the pulleys.

POWER TRANSMISSION. 155

Example. — We have a driving pulley 4 feet in diameter, and a driven
pulley 1 foot in diameter ; how near may they be placed together ? The
difference between the diameters of the two pulleys is 3 feet. We add
this to one and a-half times the diameter of the large pulley — 3 + (1 J x 4)
= 9 feet. The pulleys should therefore be placed not less than « feet
apart. If the pulleys had each been 4 feet in diameter, the difference
between their diameters would be nil, so that the minimum centres would
be 1 J X 4 = 6 feet.

When ropes are used for the transmission of power, ordinary
fast and loose pulleys are useless, as the ropes cannot be passed
from one to the other as is possibliB with a belt. To overcome
this difficulty the pulley carrying the ropes can be made either
fast or loose by means of a clutch. . A plain pulley dog-clutch is
one in which the pulley is free to revolve on the shaft, but
adjoining it is a "dog" which cannot revolve on the shafts but
is free to slide ajong it. This dog has projections which, when
it is pressed up against the pulley, engage with corresponding
projections, and causes the pulley to revolve. When the dog is
withdrawn the puU^y remains stationary and the shaft revolves.

An expanding clutch is one in which one portion of the, pulley
carrying the rim. is free to revolve, while the other portion,
which is fixed to the shaft, is arranged so that its diameter
can be, increased or diminished. • When .the diameter, of this
portion of the pulley .is increased, ;it. grips the inside of the
rim : of the other portion, 9»nd so pompels it! to rotate.

Another clutch is similar . in principle to the :dog-clutch, but
the dog is made of large diameter, and both it and the loose
portion are provided with a large number of steel wire bristles
similar to hair brushes.. When the two portions of the clutch
are brought together the bristles engage with one another, and
the dog portion of the clutch compels the other portion to
revolve. The object of this clutch is to avoid shock if the
pulley requires to be coupled or uncoupled while running. The
author's experience with this form of clutch is that it must be
of very ample size for the work, otherwise the wire bristles do
not last.

Loss of Power in Transmission. — The loss incurred in the
transmission of power by means of belts, ropes, and shafting is
considerable. It is usually considered that a belt drive absorbs
about 5 per cent, of the power transmitted, and a rope drive
about 7 per cent. In one case which came to the author's notice,
a steam engine engaged in driving a portion of an engineer's
machine shop, indicated 80 H.P. when all the tools were working.
When none of the tools were at work, and the engine was merely
driving the shafting and belts, the engine indicated 30 H.P.
The power lost in engine friction was probably about 8 H.P., so

156

MECHANICAL ENGINEEBINQ FOR BEGINNERS.

that the remaining 22 H.P., or 27^ per cent, of the power
developed, was spent in transmission.

This loss of power has often raised the question of electric
transmission, but the high first cost of the dynamo and motors
generally puts this method of transmission out of the question,
even if the gain were shown to be considerable. In point of
fact, the gain is not very great, as only 90 or 92 per cent of the
power put into the dynamo is given out as electric energy, and
only 88 to 90 per cent, of the electrical energy is restored by the
motor in the form of mechanical energy ; there is consequently a
loss of about 20 per cent., without taking into account the loss of
power in connection with those tools which still require fast and
loose pulleys and belts. If we assume that these absorb 7^ per
cent, of the power, we are no better off through transmitting the
power electrically than by doing so through shafting and pulleys.
It would certainly be too expensive, even if it were desirable, to
fit a small motor to every machine; the efficiency of small motors
is nothing like so great as it is in those of large size. In cases,
however, where power has to be transmitted to considerable
distances, electric transmission is frequently advantageous.

Shafting. — The shafting which transmits the power and
carries the pulleys is usually made of mild steel ; the lengths
are joined together by cast-iron couplings.

The power which may be transmitted by a steel shaft can be
obtained from the following figures : —

6-25

If
10

2
16

2J
30

3
50

120

5
235

400

IDia. of shaft,
j in inches.
1 H.P. per 100
/ revs.

The power which a shaft will safely transmit varies directly
as the speed ; the powers given above are for a shaft running at
100 revs, per minute, so that, if the shaft runs at 200 revs., twice
the power will be transmitted. If we wish to ascertain what
power a 2^-inch shaft running at 225 revs, will transmit, it is
only necessary to multiply 30 by 225 and divide by 100 ; the
answer is 67*5 H.P.

The above figures will be found to correspond very nearly with
the best present-day practice, but in any extreme case common-
sense must be used. For instance, if we wish to transmit 6J
H.P. through a shaft several hundred feet long running at 100
revs., we should probably decide to use a l^inch or even a 2-inch
shaft (rather than one 1 J inch diameter) for the sake of sti&ess.

POWER TRANSMISSION. 157

and to prevent the shait whipping. Whereas, if the power had
to be transmitted through a shaft a few feet long, a 1 J-inch shaft
would be sufficient. The strength of a shaft varies as the cube
of its diameter, so that if the reader wishes to ascertain how
much power can be transmitted through any shaft, the diameter
of which is not given above, it is easy for him to do so. Suppose
we wish to know how much power can be transmitted through
a shaft 10 inches in diameter, running at 100 revs, per minute.
We see that a 6-inch shaft will transmit 400 H.P. at this
speed; the cube of 6 inches is 216, and the cube of 10 inches
is l,oOO; we therefore have a simple proportion sum — thus,
216 : 400 :: 1,<»00 : a?. The answer is 1,850; a 10-inch shaft will
therefore transmit 1,850 H.P. at 100 revs., 3,700 H.P. at 200
revs., 4,625 H.P. at 250 revs., and so on. The rule gives results
a little on the safe side, especially in the case of large shafts; for
instance, the engineers of the first turbine installation at Niagara
provided a 10-inch shaft next the turbine for transmitting 5,000
H.P. at 250 revs., whereas by our rule we should only have
allowed 4,025 H.P., or have made the shaft 10^ inches diameter.
However, the shaft journals some distance away from the
turbines were made 1 1 inches diameter, probably for the sake of
stiffness.

The figures given apply to shafts which are subject to torsion
in one direction only, and the reader is warned that he must not
apply the rule to propeller shafts or to the crank-shafts of steam
and gas engines. These shafts are subject to shock and stresses
which are not easily calculated ; the only safe guide as to tho
right proportions of a crank-shaft is actual experience. A well-
known firm of engineers, making a very successful gas engine of
large powers, has found it necessary, on account of breakages,,
continually to increase the diameter of its crank-shafts beyond
the sizes which calculation would appear to render necessary,
until the crank-shafts are now half the diameter of the piston I
A somewhat similar experience befell a firm of high-speed engine
makers, who many years ago suffered from broken crank-shafts.
The directors of this firm were told by the work's manager that
the shafts were too weak, and that they ought to be one-quarter
the diameter of the L.P. piston. The manager was told that
such a rule of thumb was perfectly ridiculous, and that the
stresses could be calculated without any difficulty. After many
breakages, further calculations were made, enormous factors of
safety being apparently allowed, the shafts were strengthened,,
but still they broke. The shafts were again still further
strengthened, and finally the breakages ceased; but, curiously
enough, the dimensions of nearly all the steam-engine shafts now

158 MECHANICAL ENGINEEBING FOR BEGINNERS.

made by this firm approximate very nearly indeed to the works'
manager's despised rule.

On the face of it, a rule which does not take into account the
mean pressures, inertia of working parts, length of stroke, &c.,
does look ridiculous, but, upon going more closely into the
matter, it will be found that one factor is probably counter-
balanced by another ; for instance, the L.P. piston of a non-
condensing engine, having a late cut-off, may have a much
greater mean pressure upon it than the L.P: piston of a con-
densing engine having a fairly early cut-off, but the former will
have a better cushion to absorb the inertia of the working parts
than the latter. Again, at first sieht, one would say that a
slow-speed engine, the L.P. cylinder of which is 20 inches in
diameter by 30 inches stroke, would require a larger crank -shaft
than an engine having the cylinder of the same diameter, but
with a 10-inch stroke, as not only is the crank of the former
engine three times longer than that of the short-stroke engine,
and the torque on the shaft correspondingly greater, but the
weight of the parts of the long-stroke engine will also be greater,
owing to the greater length of connecting-rod. Against these
considerations must be set the fact that the rapid alternations of
stress in the crank-shaft of an engine, running at the high speed
which would be expected from an engine having a 20-inch by
10-inch Ii.P. cylinder, are much more punishing to the shaft
than any stress set up in the shaft of the slow-ispeed engine.
As a fact, in actual practice, the crank-shafts of long-stroke,
slow-speed steam engines are usually made about one-tifth the
diameter of the L.P. cylinder, while the crank-shafts of short-
atroke, high-speed engines require to be made about one-fourth
the diameter of the L-P. cylinder, if they are to have a reason-
Able life.

Bearings. — The bearings which carry the shafting are called
plummer blocks ; they contain a top and bottom brass which can
be renewed when much worn. The plummer blocks are usually
oarried on cast-iron brackets; when suspended from a joist or
beam the brackets which carry the plummer blocks are called
hangers. It is important that all plummer blocks should be
accurately aligned, and that the brasses should be properly
lubricated in order to minimise friction as much as possible.
Hangers are sometimes made with adjustable devices, by means
of which the bearing may be raised, lowered, or made to swivel.

Q-earing.— Before the advent of rope driving, it was usual to
transmit large powers by means of gearing. For instance, a
large mill engine would transmit by gearing its power to a
vertical shaft running the whole height of the mill; on each

POWER TRANSMISSION. 159

floor the power would be transmitted to horizontal shafts by
means of bevel wheels; the horizontal shafts would then drive
the machinery by belts.

The loss of power in transmission by gearing is less than in
transmission by ropes, but gearing is noisy and cumbrous, and a
broken toothed wheel may involve a serious stoppage of the
mill. Transmission of large powers by gearing is now seldom
resorted to, except in cases where large powers have to be
transmitted at low speeds, as in steel rolling mills.

Toothed gearing is, however, very useful in transforming
small powers at high speeds to great powers at reduced speeds,
as in the case of cranes. Thus, if a small toothed wheel or
pinion, 2 inches in diameter, drives a toothed wheel, 20 inches
in diameter, the latter will be able to raise by a drum a weight
ten times greater than would be possible if the weight were
being lifted by a drum fixed directly to the 2-inch wheel. The
weight will, however, be lifted ten times more slowly, so that
what is gained in power is lost in speed.

It is essential that the teeth of gear wheels should be machine
cut and accurately formed, if the gear is to work quietly and to
waste little power.

Toothed wheels are called spur wheels when their shafts are
parallel and they drive in the same plane. When a small wheel
drives a large one the small wheel is called a pinion.

When one wheel drives another at right angles to it, or at an
angle slightly greater or less than a right angle, the wheels are
called bevel wheels.

When the sides of a wheel are carried up so as to support the
ends of the teeth, the wheel is said to have shrouded teeth. But
one wheel only out of a pair can have shrouded teeth if carried
up to the top of the teeth. Both wheels can have shrouded
teeth if the shrouding ends just below the pitch line. A wheel
with shrouded teeth is stronger than one with plain teeth, but
the teeth cannot be machine cut.

Helical wheels are those in which the teeth, instead of running
straight across the face, are placed so that every tooth forms two
sides of a triangle, the apex of the triangle being at the centre
of the face. The effect of this is to give more surface, so that a
wheel of a given width with helical teeth will transmit more
power than one with plain straight teeth, and the action is
smoother. Teeth of this form cannot be machine cut unless the
two halves of the teeth are separated by a small space.

Worm Gearing. — Fig. 60 shows a worm and wheel; the
worm has a single spiral, or thread, with a 1-inch pitch — t.e.,
1 revolution of the spiral will move forward the teeth engaged

160

MECHANICAL ENGINEERING FOB BEGINNERS.

with it 1 inch. The wheel has twenty teeth of the same pitch,
so that it will take 20 revolutions of the worm to make the

Fig. 60. —Worm wheel.

Fig. ei.—Worm wheel.

wheel revolve once. With a single spiral it is not practicable to
effect a reduction of speed greater than 15 to 1, and 20 or 25 to

POWER TRANSMISSION. 161

1 is a better proportion. Such a reduction is too great for most
purposes.

Fig. 61 shows what the effect would be of a 3 to 1 reduction
with an ordinary worm. The pitch of the worm would have to
be very great, so that, for every revolution, it would turn the
wheel through one-third of a revolution. The result would be
that the worm would drive one of the teeth until it was out of
mesh, but the next tooth would not then be far enough round to
engage. The difficulty can, however, be got over by putting
more spirals on the worm, as shown by dotted lines, and making
a corresponding addition to the number of teeth on the wheel.
The pitch of the spirals would remain the same, and the reduc-
tion of speed would also remain the same.

With the ordinary worm and wheel, as shown by Fig. 60, the
wheel will not drive the worm ; but, with a worm having a very
coarse pitch, the wheel will drive the worm and vice versd.
Worms with a coarse pitch and several spirals are used in motor
cars where the worm drive is employed, otherwise the car could
not move without the engine being turned by hand or being run
by petrol. With the worm drive it is important to make the
effective contact surfaces as large as possible, so as to get a large
area of lubrication.

Skew wheels are constructed on the same principle — i.e., a
worm having many spirals, or, rather, parts of spirals — as the
skew wheel is not sufficiently wide for a single thread to run
right round it, as in the case of a worm; this, of course, is
immaterial so long as the pitch is right.

Gear wheels, where great strength is required, are made of
steel, and in high-class work the teeth are cut out of the solid.
In large slow-running wheels the teeth are sometimes cast of
the desired shape and trimmed up by hand. Toothed wheels
for machine tools are usually made of cast iron and carefully
machined. Such wheels work more smoothly than steel wheels.

Within recent years raw-hide pinions have been introduced to
get over the noise and jar caused by a pair of wheels running at
high speeds; they appear to answer admirably. Before these
were introduced, the author was present at an attempt made to
drive by gearing a dynamo which absorbed about 60 horse-power
and required to run at 900 revolutions per minute, this speed
being, of course, too high to admit of coupling the dynamo shaft
directly to the engine shaft. The engine ran at 380 revolutions
per minute and drove the dynamo through a pair of machine-cut,
steel wheels, but the noise made was so appalling that this
method of drive had to be abandoned.

A good method of transmitting moderate powers at fairly high

162

MECHANICAL BKGIKEBRIKG FOR BEGINNERS.

speeds in cases where belts are not admissible, and gearing is too
noisy, is by the Hans Reynolds chain, shown by Fig. 62. With
this chain a perfectly vertical drive is permissible. At a large
engineering works in the Midlands, each main line of shafting
running down the works is driven by this chain from a motor
placed directly underneath the shafting. The motors run* at a
speed considerably higher than that of the shafting.

62. — Hans Reynolds* silent chain.

With regard to the horse-power transmitted by gearing, it is
not possible to give a simple rule, as in the case of transmission
by belts and ropes. A formula often used, and which was
originally published by Messrs. Musgrave, is as follows : —

H.P. = P2xBxV.
H.P. = P2xBxV-

l,000 for cast iron,
625 „ steel;

POWER TRANSMISSION. 163

where P = circumferential pitch of teeth in inches.
B = breadth of wheel in inches.
V = velocity of pitch line in feet per minute.

This formula, however suitable for toothed wheels of fairly
large size, is not suitable for small wheels — i.e., in cases where
the pitch is 1 inch or less. Let us see how it applies to a small
toothed wheel used in the gear-box of a 15-H.P. motor car.
The wheel we will take has a 6|-inch-diameter pitch circle ; it
has twenty teeth of 1-inch pitch, its breadth is 1^ inches, and the
velocity of the pitch line is 1,500 feet.

12 X 1-25x1,500 ^ ^^

625 = ^ ^-^•'

so that by the formula the wheel would only transmit 3 H.P.

In actual practice the wheel transmits 15 H.P. or more. An
■empirical formula, given by Mr. Box, for the strength of teeth is
JSLS follows : —

S = P X W X 350,

where S = safe load on one tooth in lbs.
P = pitch of wheel in inches.
W = width of tootli „

If we apply this to the wheel under consideration, we get

1 X 1-25 X 350 = 437 lbs. safe load on one tooth. Let us see

, . ^, . .11 • 437 lbs. X 1,500 feet .q n xr p
what power this will give us. ^^^ = 19-8 H.r.,

SO that, by this rule, the wheel which is used on a 15-H.P. motor
car will safely transmit 19*8 H.P. This formula is unsuitable for
wheels in which the pitch of the teeth is much greater than 1 inch.

A beginner, for whom this book is intended, is hardly likely to
be called upon to design gear wheels for some time, and, as a
fact, there are in most drawing offices where such wheels need to
be designed some available data as to sizes, strengths, &c. ; such
data, if intelligently used, are of far more value than any empirical
formulae. If in doubt as to the strength of a gear wheel, it is a
good plan to assume that one tooth must be sufficiently strong
to transmit the whole of the power, and to see what pressure in
pounds will come upon it. This pressure will, of course, be
found by the following formula : —

H.P. X 33,000
V '

where V is the velocity in feet per minute of pitch circle.

Knowing the section of the tooth, it is not difficult to form an
•opinion as to whether it is strong enough to bear the load.

165

CHAPTER IX.
CONDENSING PLANT.

Condensers, although differing in type, are all designed with
the same object — viz., to extract from the steam the heat
remaining in it after the former has done its work, so that it
condenses and a vacuum is formed, thus relieving the engine or
turbine from the necessity of discharging the steam against
atmospheric pressure. Means must be provided for getting rid
of the condensed steam without permitting air to get into the
condenser, and so impair the vacuum.

The condensers most frequently used are either jet or surface
condensers. In an ordinary jet condenser the cooling water is
admitted in the form of a jet or spray, as shown by Fig. 63.
The exhaust steam from the engine, coming into contact with
this spray of cool water, immediately condenses, and a vacuum
is created. When the vacuum is once formed, the condenser
will continue to draw in the injection water (owing, of course,
to the pressure of the atmosphere on the surface of the water
outside), so that, unless the water has to be lifted more than 10
or 12 feet, a pump to supply the condenser with injection water
is not required.

With a jet condenser, in which the water mixes so intimately
with the steam, a smaller quantity of water is required to con-
dense the steam than is necessary in a surface condenser where
the water has to effect its cooling action through tubes ; but, on
the other hand, if it is desired to use the condensed steam over
again for feeding the boiler, it is necessary for the cooling water,
as well as the feed water, to be free from impurities which may
be injurious to the boiler, for, as we have said, the injection
water mixes with the exhaust steam.

The amount of injection or cooling water required for a jet
condenser is about 25 or 30 times the weight of the steam to be
condensed. The size of a jet condenser is not of great im-
portance ; it is usually made about three-quarters the capacity of
the IaP. cylinder. The shape, too, is of but small importance.

With a jet condenser fixed at, or below, the level of the engine,
a pump is required to remove the condensed steam, cooling water,
and a certain amount of vapour from the condenser; such a pump

166

MECHANICAL ENGINEERING FOB BEGINNERS.

is called an air pump, as it prevents the air from getting into the
condenser, also to distinguish it from a circulating pump.

^ToAir
Pump

Fig 63. — Jet condenser.

An air pump is shown by Fig. 64. The pump is shown with
head valves, bucket valves, and foot valves, but in practice the
foot valves are now often dispensed with. The Edwards air

CONDENSING PLAIJT.

167

pump, which has neither bucket valves nor foot valves, will be
described later.

Surface Condensing Plant. — In a surface condenser the
cooling water is kept separate from the steam which it has to
condense. The water is passed through a large number of tubes,
usually about | inch outside diameter, made of Muntz metal or
of an alloy composed of 70 per cent, copper and 30 per cent. zinc.
A surface condenser is shown by Fig. 65. A condenser of this

< Access Doer
Fig. 64. — ^Air pump.

type is always used on board ship, where it is necessary to use
the condensed steam over and over again for feeding the boilers,
and where it is inadmissible to mix sea water with the feed
water. A surface condenser is also used on land in cases where
the cooling water is not suitable for use in the boilers.

With a surface condenser the air pump may be considerably
smaller than with a jet condenser, as it has to deal with the
condensed steam and vapour only; a separate pump, called a
circulating pump, is used for circulating the water through the

168

MECHANICAL ENGINEERING FOR BEGINNERS.

tubes. The circulating pump may be similar to the air pump
shown by Fig. 64 ; but pumps of the centrifugal type, in which
there are no valves, are frequently used. In a centrifugal pump
(illustrated in a later chapter) there is an impeller, which is
rotated at a rapid rate, and imparts sufficient motion to the water
to make it travel through the tubes of the condenser.

Extent of Cooling Surface. — In a surface condenser there
should be 1 square foot of cooling surface for every 9 to 10 lbs.
of steam to be condensed per hour, assuming the temperature of
the cooling water to be about 60" to 70° F. In the case of
condensing plant for a steam turbine where it is desirable to get

Circuiatmg

Water

Dischar ge

Exhaust

Stesm

Circuiating
Water Inlet

Discharge to
Air Pump.

Fig. 65. — Surface condenser.

an extremely good vacuum, a square foot of cooling surface is
frequently provided for every 6 to 8 lbs. of steam to be con-
densed.

The tubes of a surface condenser should be of a length not
exceeding 12 feet, unless supported in the centre. Each tube is
free to slide in its hole in the end plate, the joint being made
by a small brass ferrule and cotton rope packing, as shown by
Fig. 66. If the tubes are screwed or expanded into the end
plates they are not able to expand or contract, and the condenser
does not remain air tight.

It has recently been found that by dividing a surface condenser

CONDENSING PLANT.

169

into sections horizontally and draining away the water from each
section, a smaller cooling surface is equally efifective. It is
believed that a thick film of water hangs
round the tubes, and prevents the conduc-
tion of heat through them. If the water
is drained away from the upper rows of
tubes as it is formed, the lower tubes
remain fairly dry, and are much more
effective.

Amount of Cooling Water Required.
— The amount of cooling water required
in connection with a surface condenser is
variously stated in engineering pocket-
books and text-books to be from thirty
to seventy times the amount of feed water.
The amount of cooling water required depends on its tempera-
ture, but the following table, showing the actual vacua obtained
with different quantities of feed water at a temperature of 65" F.
may be useful. The table has been prepared from curves given
by Mr. Allen in his paper upon condensing plants read before
the Institution of Civil Engineers in 1905.

The curves themselves were plotted from a very large number
of experiments carried out by Mr. Allen : —

Fig. 66.— Tube end and
ferrule, half size.

Cooling Surface = 1 Square Foot for
6 Lbs. of Steam Condensed.

Cooling Surface = 1 Square Foot for
10 Lbs. of Steam Condensed.

Amount of Cooling
Water.

Vacuum.

Amount of Cooling
Water.

Vacuum.

40 times feed.

27-2"

27 r

28-0"
28-25"

40 times feed.

26-75"
27-26"
2775"
28-0"

From the above figures it will be seen that for a vacuum of
26 inches or 27 inches, forty times the amount of the feed is
sufficient, but if very high vacua are desired the amount of
cooling water must be increased, or water of a lower temperature
used.

The air pump in the experiments in question had a capacity
of '75 cubic foot per pound of steam condensed. Temperature
of cooling water 65" F., Barometer 29*9.

Capacity of Air Pump. — The capacity of the air pump — i.e,,
the volume swept by the bucket multiplied by the number of

170

MECHANICAL ENGINEERING FOB BEGINNERS

strokes per minute — should be from -75 to 1 cubic foot per pound
of steam to be condensed. With some surface condensing plants
a pump capacity of 1-5 cubic feet per pound of steam to be
condensed has been allowed ; it is, however, useless to provide
an air pump of very large capacity, unless the temperature of the
cooling water is fairly low, and the condenser has adequate
cooling surface. For example, assuming the condenser is unable
to reduce the temperature of the exhaust steam below 105** F.,
no air pump, however large, can give a better vacuum than
27*7 inches (see the subjoined table), for at this temperature and
pressure water turns into steam. If an attempt is made to get
a higher vacuum than this by means of the air pump, not only
will the exhaust steam not condense, but any water lying in the
condenser will vaporise, and, as the pump will be quite unable
to cope with such an enormous volume of steam, the vacuum
will fall to a point at which steam will condense at a temperature
of 105" F.

TABLE XVI.

Degrees
Fahr.

Vacuum in

Inches when

Atmospheric

Pressure

= 14-7 Lbs.

Absolute

Pi-essui'e in

Inches of

Mercury.

Absolute
Pressure.

Total Heat in

1 Lb. of Hteam

from 32° F.

Volume of
1 Lb. of
Steam.

Lbs. per sq. in.

B.T.U.

Cubic feet.

29-7

•181

•089

1091-2

3,226

29-6

•362

•178

109rt-6

1,696

29-4

•517

•264

1099-7

1,220

29-2

•733

•360

1102-8

877

28-9

1024

•603

1106-8

641

28-5

1-410

•693

1108 9

549

100

1917

942

1111-9

363

106

27-7

2-229

1^096

1113-4

307

From this table it will be seen that, in order to obtain a
vacuum of 28 inches, the temperature of the steam must be
reduced below 100** F. To obtain a vacuum of 29*6 the steam
must be cooled below 50" F.

If the atmospheric pressure is higher than 14^7 lbs. (or 29^95
inches of mercury) then a slightly better vacuum can be obtained
with the above-mentioned temperatures. The vacuum it is
possible to obtain theoretically with given temperatures, and
with the barometer standing higher or lower than 29-95 inches
can be ascertained by deducting the pressure given in inches of
mercury (column 3) corresponding with the temperature of the
steam, from the height of the barometer in inches of mercury.

CONDENSING PLANT. 171

Some engineers employ two air pumps with a surface condenser,
one a dry air pump for carrying away the vapour, and the
other a wet pump for removing the water. The former is
connected to the top of the condenser (the vapour preferably
being cooled before admission to the pump), and the latter to the
lowest part of the condenser. It is somewhat doubtful, however,
whether the advantage gained outweighs the additional cost
and complication involved.

Vacuum Augmenter. — In order to assist the air pumps to
deal with vapour Mr. Parsons has introduced what he calls a
vacuum augmenter. This is a small apparatus fitted between
the condenser and the air pump, and through which a jet of live
steam is blown; this jet draws out considerable quantities of
vapour from the pipe to which it is fitted, compresses it, and
delivers it to the air pumps.

In striving for a high vacuum there is one point which must
not be overlooked, it is this — It is quite useless to have a very
high vacuum in the condenser if, owing to the want of area of
the exhaust ports, a correspondingly high vacuum is not obtained
in the cylinder of the engine. The reader will see from the
last column of Table xvi. how enormous is the volume of a
pound of steam at very low pressure (or high vacuum), and will
realise how difficult it must be to get such a volume of steam out
of a cylinder through its ports.

In Lancashire it has been found from experience that a
vacuum of 26 inches gives the most economical results. The
explanation doubtless is that, if a higher vacuum is obtained in
the condenser, the exhaust steam must be cooled down to a much
greater extent, and the temperature of the condensed steam which
is fed into the boiler is correspondingly reduced. This reduced
temperature of the boiler feed probably neutralises the gain due
to a slightly better vacuum in the engine.

Corrosion of Condenser Tubes. — The galvanic action set
up by Muntz metal tubes, brass tube plates, and the cast-iron
shell tends to make the latter corrode. This in itself is not
very harmful if the shell is made fairly thick in the first place,
but if a piece of rusty iron lodges in one of the tubes it quickly
corrodes its way right through. To overcome this source of
trouble, Mr. Edwards advocates increasing the speed at which
the water travels through the tubes, so as to sweep them more
effectually. The usual speed is about 300 feet per minute ; by
greatly increasing this velocity Mr. Edwards claims that he
has effected an improvement in the life of the tubes. Coating
the inside of the condenser body with a wash of cement has
proved very useful in preventing corrosion.

172

MECHANICAL ENGINEERING FOR BEGINNERS.

Edwards' Air Pump. — A pump which has come very largely
into use during the last few years is Edwards' air pump, as
shown by Figs. 67 and 68. This air pump differs from those of
the older pattern, in that it has no bucket valves or foot valves.

and runs at a considerably higher speed than was formerly
considered practicable.

The pump is placed below the level of the condenser, so that
the condensed steam flows continuously by gravity from the

CONDENSING PLANT. 173

condenser to the base of the pump. On the downward stroke
a vacuum is created between the bucket and the head valves,
and at the bottom of the stroke the bucket uncovers a row of
ports, so that the vapour from the condenser enters the barrel.
By its conical shape the bucket projects the water which was
lying in the base of the pump through the ports into the pump
barrel ; the water is projected with considerable force, and it
also entrains a certain amount of vapour. On the up stroke the
bucket closes the ports and sweeps out the water and vapour,
through the head valves.

This pump has many advantages ; in the first place, the bucket
valves and foot valves, which, in the old form of pump, were
necessarily rather inaccessible, are eliminated. In the second
place, when the bucket uncovers the ports the vapour has free
entry to the barrel, whereas, in the old form of pump, a certain
pressure in the condenser was necessary to open the valves. In
the third place, owing to the high speed at which it is possible
to run the pump, it deals with small quantities of water at a
time, and runs very smoothly and with freedom from shock.
There is a large inspection door, shown at the top right-hand
side of the illustration, which gives access to the head valves.

Evaporative Condenser. — In cases where water for con-
densing purposes is scarce, or has to be paid for, and the amount
of steam to be condensed is comparatively small, an evaporative
condenser is sometimes employed. This form of condenser
consists of a range of pipes having external gills, through which
pipes the steam is passed. Water is allowed to trickle on to
them, and the water is evaporated, thus extracting a considerable
amount of heat from the pipes. By this system, instead of 30
to 40 times the amount of feed-water being required for con-
densing purposes, an amount of water equivalent only to the
feed is required. The remarks in connection with power brakes
in Chap. vi. will make the reason for this clear. The objection
to this form of condenser is that it takes up a good deal of room,
and the clouds of steam arising from it are undesirable in a town.

The syector Condenser. — In this condenser the exhaust
steam and the cooling water are mixed together, and no air
pump is required. The principle upon which the ejector con-
denser works is somewhat similar to that of the injector
previously described, but, instead of water being fed into a
boiler, the cooling water and condensed steam are discharged
against atmospheric pressure into a hot well. Unlike the
injector, the nozzle through which the steam passes is per-
forated with a large number of openings through which the
water comes in contact with the exhaust steam and condenses it.

174 MECHANICAL EXGINEERINQ FOR BEGINNERS.

The ejector condenser to be really reliable should be supplied
with cooling water from a tank 20 feet above it, or under a
pressure of about 10 lbs. per square inch if supplied direct from
a centrifugal pump. The advantage of this form of condenser is
that it will discharge the condensed steam against atmospheric
pressure while maintaining a vacuum of 24 or 26 inches in the
engine without the aid of an air pump.

The ejector form of condenser is therefore very inexpensive,
and no power is expended in driving an air pump. Such a
condenser is, however, a little extravagant in cooling water ;
about 40 times the weight of the steam condensed is required,
and the same quantity of water is required at light as at full
loads. A non-return valve is invariably fitted between the
engine and the condenser, but, even with this safeguard, there is
a certain element of risk — viz., that of the water finding its way
into the engine.

The danger can be avoided by carrying the exhaust pipe from
the engine about 32 feet upwards and down again to the
condenser, as water will not rise to this height under the
vacuum formed. If, however, the pipes are carried up to such
a height, a plain jet condenser of the form shown by Fig. 63
may be used. This would then be called a barometric condenser.

Barometric Condenser. — If a condenser, as shown by Fig.
63, is placed 35 feet above the level of the water into which it
discharges, it will free itself of water by means of gravity. The
pressure per 'square inch at the bottom of a column of water is
•433 lb. for every foot in height, so that a column of water 34 feet
high exerts a pressure of 14*7 lbs. per square inch. Even if a
perfect vacuum were formed in the condenser, the atmospheric
pressure outside would not force the water to a height greater
than this. If, then, the jet condenser condenses the steam at,
say, 35 feet above the level of the discharge, the water will flow
away by gravity, and the vacuum will still be maintained.

It has been found that the drops of injection water entrain
any vapour, and that the velocity at which the water descends
the pipe is sufficiently great to carry the bubbles down with it.

With this form of condenser no air pump is required, merely
a centrifugal pump to assist in raising the cooling water to the
required height. The amount of cooling water required is the
same as in a jet condenser fixed in the ordinary way — viz., about
25 times the weight of the steam condensed.

Cooling Towers. — The chief difficulty in connection with the
use of condensing plant in large towns has been the question of
water supply. We have seen that the water required to con-
dense the steam in a surface condenser is 40 or more times the

CONDENSING PLANT. 175

weight of the steam used, so that in a large power installation
the amount of cooling water required is very great indeed.

In the case of Lancashire cotton mills which are not on the
banks of a canal, it has been customary to construct a fairly
large reservoir of water, known as a lodge, from which the
cooling water is drawn, and to which it is returned after passing
through the condenser. The surface of the water being exposed
to the atmosphere gives up a certain proportion of the heat
extracted from the steam, and although towards the end of the
day the temperature of the water, especially in hot weather,
becomes rather high, yet the system is found to answer fairly
well.

This method of cooling the water, although suitable in the
case of a mill where the horse-power rarely exceeds 1,000 or
1,200, is not suitable, on account of the large size of reservoir
required and the expense of the land needed, for a large power
installation where many thousands of horse-power are developed.

The plan now usually adopted is to employ cooling towers.
These towers, which range from 40 to 80 feet high, are filled
with %ome material suitable for breaking up a mass of water and
exposing as much of the surface as possible to the atmosphere.
In one make of tower a large number of short earthenware
pipes are used ; they are stood up end to end, but the openings
of the pipes do not come exactly over one another, thus the
downward stream of water is continually broken up. The water
from the condenser is pumped up to the top of the tower and
trickles down the sides of the pipes, while a current of air rises
up and meets it and extracts a good deal of heat from the water.

In another make of tower, galvanised wire and timber slats
are used. Originally fans were used to send a current of air up
the inside of the towers, but it has been found that in many
cases the diiBference between the temperature of the air inside
and outside the tower is quite sufficient to cause a good draught
of air, and that a fan can be dispensed with.

The tower stands over a small tank or reservoir formed of
concrete, into which the water is allowed to fall after passing
down the tower. In large installations several towers are used.

A question which may occur to the student is — Why not
dispense with water altogether and use air as the cooling
medium in the first place 1 The reply is, that it is not possible
to do so, as the specific heat of air is too low, and the volume of
air required would be excessive. As nothing has yet been said
about the specific of heat of substances, a little digression must
be made.

Speoiflo Heat is the amount of heat required to raise 1 lb.

176 MECHANICAL ENGINEERING FOB BEGINNERS.

of the substance through 1" F. Thus 1 British thermal unit
will raise 1 lb. of water at its greatest density through 1' F.
The specific heat of water is therefore said to be 1, and is
genei'ally adopted as the standard of comparison.

The specific heat of air is *238 and of cast iron *13, so that
the specific heat of both air and cast iron is less than that of
water ; or, in other words, less heat is required to warm 1 lb. of
either by 1" than is required to warm the same weight (not
volume) of water. Conversely, a gas, such as 9ir, the specific
heat of which is low, is less capable of abstracting heat from
another substance or gas, with a given rise of temperature, than
a body the specific heat of which is high, such as water. To
return to the condensing question.

The specific heat of air is -238, so that if air were of the same
weight as water, 4*2 times more air than water would be required
for condensing. A cubic foot of air at 32" F., however, weighs
only '08 lb., while a cubic foot of water weighs 62*4 lbs., so that
water is 780 times heavier than air. We should, therefore,
require a volume of air 780 x 4*2, or 3,270 times greater than
that of water, to obtain the same cooling effect. It would be
out of the question to deal with such an enormous volume of air.

While upon the subject of specific heat, a few words on the
subject of the specific heat of steam may be said, as this is of
practical interest when dealing with superheated steam. The
specific heat of steam was originally found by Regnault to be
about -48, and this figure is usually taken to be correct, although
doubts have often been expressed as to its accuracy. Some
experiments recently made at the Munich Technical School
have thrown a good deal of light upon the subject. From
the curves reproduced in Engineering, vol. Ixxxiii., p. 227, it
would appear that the specific heat of saturated steam varies
from -45 at atmospheric pressure to '62 at 142 "24 lbs. pressure,
but as soon as the steam begins to get superheated the specific
heat falls, until at 500° F. the specific heat varies from -46 to
•6, according to the pressure, while at 600* F. the specific heat
varies from -475 to -498.

177

CHAPTER X.
THE STEAM TURBINE.

The great success which the steam turbine has achieved during
the last few years seems to render it probable that before
many years are past it will largely, if not entirely, supersede
reciprocating engines for marine work and for driving electric
generators. Instead of huge engines having big pistons, with
heavy piston- and connecting-rods moving up. and down and
turning a crank, we have in a turbine a revolving drum receiv-
ing its motion direct from the steam.

The credit for this transformation is almost entirely due to
the Hon. C. A. Parsons, who first believed in the possibility of
constructing a turbine to give considerable power without an
excessive consumption of steam. The first Parsons turbine of
about 10 H.P. was constructed in 1885, and the first compara-
tively large turbine in 1890; and although during the next
few years a fair number of turbines were made and supplied,
their introduction was a stiff uphill fight. The earlier turbine
undoubtedly used more steam than a good reciprocating
engine, and the makers of such engines made the most of the
fact; the turbine was referred to as a steam eater, and many
were the jokes made as to its capacity in this respect. It was
not until the Parsons turbine was worked in connection with a
condenser that it was able to compete on terms of equality, as
regards consumption of steam, with a reciprocating engine.
When the figures obtained at the first authentic condensing
turbine trial were published they were received with a certain
amount of incredulity, but from that date the advance of the
turbine into general favour has been steady and continuous.

The Parsons turbine is of the parallel flow, reaction * type—
i.e., one in which the steam flows through the turbine in a
direction parallel with its axis, as shown by Fig. 69. The
illustration shows a Parsons - Willans turbine (the difference
between this and the original Parsons turbine is explained
later) without bearings or governor. The blades in Fig. 69
are shown diagrammatically — i,e,j a row of blades is represented
by one line ; they are shown in detail by Figs. 70 and 72. The

* The meaning of the term reaction turbine is explained in the Chapter
on Water Turbines.

178

MECHANICAL ENGINEERING FOB BEGINNERS.

rotating drum or rotor is provided with a large number of
blades, and between each row of moving blades there is a row
of fixed guide blades attached to the casing, as shown by Fig. 70,

Steam at high pressure is admitted at A, Fig. 69. It passes
through the guide blades and impinges on a row of blades
attached to the drum ; it exerts a reactionary force on these
blades, causes them to move and thus rotates the drum to which
they are attached. After passing through the next row of guide
blades the steam impinges on the succeeding row of blades, and
so on, until the steam is fully expanded, when it passes away to
the condenser.

It will be seen from Fig. 69 that at first the blades are short,
they are also closely spaced ; as the steam expands blades of a
greater length and width, and more coarsely pitched, are used.

Dummy
Pistons

Fig. 69. — Willans-Parsons turbine.

The drum also is increased in diameter ; this allows of a larger
number of blades being used, and the somewhat weaker turning
effort of the steam on them is made up for by their increased
distance from the centre of the drum, or, in other words, by their
increased speed. By the time the steam has- reached the end of
the turbine it has, by a long series of steps, fallen in pressure
and has imparted a large portion of its energy to the rotating
drum.

In order to prevent end thrust the drum is enlarged at the
high-pressure end of the turbine ; these enlargements are called
dummy pistons, and are provided with baffle rings, as shown by
Fi^. 71. The turbine is usually arranged so that high-pressure
steam can be admitted by a pipe or passage to the low-pressure
end; this enables additional power to be obtained for short
periods, but of course uneconomically. The pipes shown under-

THE STEAM TURBINE.

179

neath the turbine are for balancing purposes. The small pipe
allows steam at a pressure corresponding with that in the middle
of the turbine to press against the face of the dummy piston,

Fig. 70.

Fig. 72.

Steam

^z///z/

S* Shrouding
F' Foundathon

C' CBuiking
Strip

BaFFIe Rings

round
Dummy Pistons.
Fig. 71.
Figs. 70 to 72.— Turbine blading.

while the large pipe places the back of the piston in communi-
cation with the condenser.

In the Parsons turbine the governor is driven from the main
shaft by a worm and wheel, and is arranged so that the steam is

180 MECHANICAL EN6INEEBIN6 FOB BEGINNERS.

admitted intermittently or in puflfs ; when the turbine is lightly
loaded the interval between the puflfe is longer than when fully
loaded. With a full load on the turbine the puffs are almost
continuous. The effect of this arrangement is that the turbine
is always supplied with steam at high pressure even when
working at light loads. If the governor worked on the ordinary
throttling system the turbine, at light loads, would be supplied
with steam at a pressure considerably less than that of the
boiler.

The consumption of steam in a Parsons' 3,500 kilowatt
turbine, say 5,100 brake horse-power, working with 200 lbs.
steam pressure, and 121° of superheat, has been as low as 13-2
lbs. per kilowatt hour. This is equivalent to 9*85 lbs. per
electrical horse-power, and about 9 lbs. per brake horse-power.
The best result obtained, within the author's knowledge, with a
triple-expansion reciprocating engine working with the same
pressure and the same degree of superheat, has been 11*9 lbs.
per brake horse-power.

The Willans-Parsons Turbine. — The principle upon which
this turbine works is practically the same as the Parsons, but
there are certain differences in the construction. In the Parsons
turbine the blades are placed separately in grooves cut in the
rotor and casing and are held in position by small pieces of
bronze which are wedged or caulked in between the blades, the
ends of the blades being free. In the Willans-Parsons turbine
the outer ends of the blades are riveted into an encircling
U-shaped ring of bronze, as shown by Fig. 70. The root of
each blade is inserted into a saw cut in a ring, one side of the
blade at the root is turned over, and the whole is wedged tightly
in its groove by a caulking strip. The blades with their rings
are put into position in sections.

The blades shown by the illustration are drawn to approxi-
mately half their actual size, and represent those used about
half way along the rotor of a 1,500 kilowatt (2,200 B.H.P.)
turbine running at 1,500 revs, per minute. In such a turbine
the blades range in length from 1 or 1^ inches at the high-
pressure end to about 6 inches at the low-pressure end. In large
marine turbines running at a considerably lower speed the blades
are much longer and wider. The blades at the low-pressure end
of one of the turbines for an Atlantic liner, the " Mauretania,"
are about 23 inches long and 2 inches wide.

The U-shaped shrouding ring at the ends of the blades is
used chiefly in the Willans-Parsons turbine (and by Messrs.
Yarrow); in the original Parsons turbine the blades were
strengthened by a ring let into the blades, and soldered to them

THE STEAM TURBINE.

181

by silver solder, as shown by Fig. 72, at the top right-hand
corner of the illustration ; if the blades were very long, two or
more rings were used.

The shrouding, besides strengthening the blades, has two other
advantages — viz., should any whipping of the shaft or hogging
of the casing take place and cause the blades to touch the casing,
the shrouding will merely rub against the latter ; the shrouding
will certainly wear away, but the blades will not be stripped
away, as is the case if the ends are unprotected. The other
advantage of shrouding the ends and having a ring round the
roots of the blades, is that the flow of steam past the ends is to a
certain extent checked, and the loss due to leakage past the ends
is probably less than with blades having free ends.

The consumption of steam is approximately the same as in the
Parsons turbine.

In some of the Willans-Parsons turbines the upper portion of
the casing is hinged, so that there is no danger of damaging the
blading when the upper half is being opened up for inspection or
during replacement. All pipes and connections are attached
to the lower half of the casing. The Willans-Parsons turbine
is governed by throttling, and not by admitting steam inter-
mittently.

The Brush-Parsons Turbine.— This turbine is very similar
to the Parsons ; it differs only in minor points of construction ;
for instance, the blades are constructed with a strengthening ring
let into their ends, instead of into one si<le as in the Parsons. A
special centrifugal water gland is used for packing the shaft at
the ends of the turbine.

The speeds at which the Parsons, Willans-Parsons, and Brush-
Parsons turbines are usually designed to run, when driving
electric generators, are as follows : —

TABLE XVII.

Electrical Output.

Brake H.P.

Ecvolutions per Minute.

600 Kilowatts.

760
1,000
1,500
3,000
6,000

730
1,100
1,460
2,200
4,400
7,300

1,600 to 3,000
1,600 „ 3,000
1,600 „ 1,800
1,600 „ 1,800
760 „ 1,600
760 „ 1,000

For marine work the speed of the turbine is much less ; for
instance, the H.P. and L.P. turbines in the Cunard Liner

182 MECHANICAL ENGINEERING FOB BEGINNERS.

" Lusitania,'' which collectively give about 64,000 H.P., run at
about 188 revs, per minute.

When the turbine is coupled direct to an alternator, the exact
speed is determined by the periodicity of the latter. The meaning
of the periodicity of an alternator is given in the electrical
chapter. The peripheral speed of the blades in turbines of the
Parsons type does not usually exceed 300 feet per second.

The De Laval Steam Turbine. — ^This turbine, which was
invented by a Swedish engineer, works on a principle different
from that of the Parsons. In the latter the steam is, as we have
seen, expanded from its highest to its lowest pressure through a
long succession of steps, each row of blades absorbing a small
part of the energy of the steam.

In the De Laval turbine, the steam is expanded in one step
from its highest to its lowest pressure. This expansion is carried
out in a nozzle. It has been proved by experiment that, although
steam loses its pressure if expanded from a small volume to a
larger volume, it does not lose its temperature or energy, pro-
vided it does no external work during such expansion.*

In the De Laval turbine this fact is made use of, so that,
instead of having a small volume of steam issuing from a nozzle
at a high pressure (in which case much of the steam would
expand in the air after striking the buckets, and its energy be
lost), the nozzles are constructed so that the steam is expanded
in them before leaving the orifice. We have, therefore, a very
large volume of steam at low pressure travelling at high speed ;
by allowing the steam to impinge on buckets, also travelling at a
high speed, we utilise a very large part of its energy. A turbine
working on this principle is called an impulse turbine. The
meaning of the term impulse turbine is explained in the chapter
dealing with water turbines.

Fig. 73 shows the ring of buckets and four nozzles of the De
Laval turbine; the illustration shows clearly the action of the
steam on the blades. In order to take the greatest advantage
of the energy of the steam, the ring of buckets must travel at
a very high speed indeed ; the peripheral speed of the buckets
should be 47 per cent, of the velocity of the steam, so that, if the
steam leaves the nozzle at a speed of 4,000 feet per second, the

* Joule's law is — *<When a gas expands without doins external work,
and without taking in or giving out heat, its temperature aoes not change.*'
To prove this rule, Joule connected a vessel containing compressed gas
with another vessel that was emptv by means of a pipe with a closed stop
cock. Both vessels were immersed in a tub of water, and were allowed to
assume a uniform temperature. Then the stop cock was opened, the gas
expanded without doing external work, and finally the temperature of the
water in the tub was found to have undergone no change.

THE STEAM TURBINE.

183

peripheral speed of the buckets should be 1,880 feet per second;
but for practical reasons the speed is considerably less.

In the case of a 300 H.P. turbine the outside diameter of the
wheel is 3l| inches; it runs at 10,600 revs, per minute, giving
a peripheral speed of about 1,457 feet per second or 87,420 feet
per minute. With smaller turbines the peripheral speed is less.

Fig. 73. — Bucket wheel of De Laval turbine.

To enable a wheel to run at this enormous speed several
interesting methods of construction have been adopted. The
wheel is of the disc form, and in the larger turbines it is solid
throughout ; there is not even a hole through the boss, the shaft
being bolted to the wheel by flanges on each side; the blades
or buckets are dovetailed into the rim. The centrifugal force
on these buckets is considerable; a bucket weighing ^ oz.

184

MECHANICAL ENGINEEBINO FOR BEGINNERS.

if rotated at a radius of 1*25 feet, will develop a centrifugal
force of about 13*4 cwts., when run at a speed of 10,600 revs,
per minute. The weight of a bucket of a 300 H.P. turbine is a
little heavier than ^ oz. The ends of the buckets form what is
practically an encircling ring ; it would be impossible to surround
the buckets (of the larger turbines) with an encircling ring of
steel, as is sometimes stated to be the case. If the reader will
work out the stress in such a ring due to centrifugal force
(the rule is given in Chapter vii.), he will find that it amounts
to about 95 tons per square inch, or sufficient to burst any steel
ring. Each bucket is firmly dovetailed into the disc, and has
only to withstand the stress due to its own centrifugal force.

Fig. 74. — De Laval turbine with gearing.

The shaft carrying the bucket wheel is of small size ; that of a
300 B.H.P. turbine is only l^\ inches diameter, the reason for
this being that it is impossible to insure that the centre of the
shaft shall be absolutely in the centre of gravity of the wheel, or,
in other words, that the wheel shall be in perfect balance. If
a very stiff shaft were used the effect of a slight want of balance
would, at the very high speeds employed, be to cause excessive
vibration. By employing a light and, therefore, somewhat
flexible shaft, the shaft is enabled to spring somewhat; when
the wheel reaches about J or ^ of its full speed, it settles itself
on a new centre so as to run smoothly and free from vibration.

The speed of the De Laval turbine, which ranges from 30,000
revs, per minute in the 5 H.P. turbine, to 10,600 revs, in the

THE STEAM TURBINE. 185

300 B.H.P. turbine, is too high to enable it to drive even a
dynamo if directly coupled to it. The speed is therefore reduced
by double helical gearing, as shown by Fig. 74. The two halves
-of the helical teeth are slightly separated, so that they can be
machine cut. The pinion is made in one piece with the shaft,
and the linear velocity of the teeth is about 1,000 feet per second.
The helical wheels are enclosed in a casing, as shown by dotted
lines. The illustration shows the turbine driving a belt pulley,
but a dynamo, pump, or fan, can be placed in the position
occupied by the pulley.

The turbine is provided with several nozzles, some of which
are usually closed when working with high steam pressures and
condensing; some of the nozzle3 are also closed when the turbine
is working lightly loaded.

A consumption of 18 -9 lbs. per kilowatt hour, working with
193 lbs. pressure, and with 60"* of superheat, has been recorded.
The De Laval turbine is made in this country by Messrs.
Greenwood & Batley of Leeds.

The Curtis turbine, made by the British Thomson-Houston
Co., is of the "Impulse" type, but is designed so that an
extremely high rotative speed is not necessary in order to
obtain good results.

In the Curtis turbine, as in the Laval, the steam is expanded
in nozzles before striking the buckets, but not to the same
extent. The nozzles are designed so that the speed of the
steam issuing from them shall be about 2,000 feet per second.
After the steam has passed through one row of buckets its
course is altered by stationary buckets or blades, so that the
steam shall impinge to the best advantage on a second row of
moving buckets. After the steam has passed through this second
row of buckets, which completes one stage, it is again passed
through nozzles, and goes through a course similar to that just
described — viz., moving buckets, stationary buckets, and moving
buckets.

Fig. 75"^ shows the arrangement of nozzles and buckets in a
two stage turbine. The two rows of moving buckets are bolted
to the upper and lower sides of one rotating disc.

Fig. 76 shows the general arrangement of the Curtis turbine
driving a dynamo, tie latter being placed immediately above
the turbine.

It* will be seen from Fig. 76 that the Curtis turbine, in its
larger sizes, has, unlike the Parsons and the De Laval, a vertical

* Fig. 76 causes a curious optical illusion, each row of buckets apparently
varying in width. Measurement by means of a pair of dividers will show
that this is not so.

186

MECHANICAL ENGINEERING FOR BEGINNERS.

shaft. This shaft, which carries the whole of the revolving
portions of the turbine and dynamo, is supported by a footstep
bearing. The latter is made in halves^; the surfaces are kept
apart by a film of oil or water supplied under pressure. The
lubricant, after leaving the footstep bearing, passes upwards, and
lubricates a guide bearing which keeps the shaft central. The
upper guide bearings at A and B are also lubricated by oil
supplied under pressure ; after use, the oil passes to a tank, and
is used over and over again.

The governing of the Curtis turbine is effected by opening
or closing the valves controlling the nozzles, as shown by
Fig. 75, the amount of steam admitted to the turbine being
proportional to the load ; the steam pressure is not reduced by
throttling.

3UAM CHEST

NOZZL£
•TATIONARy flUUU

XCCCXCCCCCCCCCC

M(MN6BMI>U|
5TATI0NAFW BLADES

MOVING euuita

cccccccccccccccccccc

I I I

1 t

Fig. 75. — Curtis turbine blading.

The buckets of the Curtis turbine are cut out of a solid ring
which is bolted to a disc. A thin encircling ring of steel
surrounds the outer ends of the buckets.

The clearances of the Curtis turbine, unlike those of the
Parsons, can be adjusted to a nicety by means of a strong screw
at the bottom of the footstep bearing. In the larger turbines
this screw is worked by a worm and wheel. The buckets being
shrouded, if any rubbing should occur the shrouding will wear,
but the buckets will not be destroyed.

The lower part of the Curtis turbine usually contains the
surface condenser. The Curtis turbine occupies much less floor
space than one of the Parsons type, and the construction of the

THE STEAM TURBINE.

187

Dy n I

Fig 76.— Curtis turbine.

188 MECHANICAL ENGINEERING FOB BEGINNERS.

buckets, cut out of the solid as they are and shrouded, is a good
feature.

The consumption of steam in Curtis turbines usually employed
for driving generators up to 3,000 kilowatts, working with super-
heated steam and with a good vacuum, is frequently about 16*5
lbs. per kilowatt hour ; but in the case of a 10,000 kilowatt five-
stage turbine, running at 750 revs, per minute, a consumption as
low as 12-9 lbs. per kilowatt has been recorded. The steam
pressure was 176 lbs. ; superheat, 147' ; vacuum, 29*47 inches.
This turbine was one of four erected at the Fisk Generating
Station, Chicago.

The Bateau Turbine. — The Bateau turbine, made by Messrs.
Fraser <fe Chalmers, is similar in principle to the Curtis, but the
shaft is horizontal instead of vertical. This turbine has been
employed to a considerable extent on the Continent, fre-
quently being driven by the exhaust steam from reciprocating
engines.

In cases where the steam is only supplied intermittently, a
heat accumulator is used. This, in its simplest form, consists of
an old boiler shell filled with scrap iron, into which the exhaust
steam is taken, and from which the turbine draws its supply.
The exhaust steam, on entering the shell, gives up some of its
heat to the scrap iron, and partially condenses ; when the supply
of exhaust steam from the reciprocating engine ceases, and the
turbine goes on drawing its steam, the pressure in the shell falls,
and the water lying in it turns into steam, and so keeps up the
supply. The turbine must work condensing; otherwise, steam at
the somewhat low pressure available would be of comparatively
little use. A more elaborate heat accumulator consists of a shell
containing shallow trays of water, or of a drum partly filled with
water, and containing tubes through which the steam passes, the
tubes being arranged so that a good circulation of water is main-
tained around them, but the principle is the same.

Westinghouse Turbine. — In this turbine the impulse
principle is combined with the reaction or Parsons system.
Steam is admitted at the centre of the turbine, and flows
outwards towards the ends. It impinges first on two impulse
wheels, as in the De Laval turbine ; after passing the impulse
wheels the steam, which has dropped to about one-third of its
original pressure, passes through blading of the Parsons type,
and leaves at each end of the turbine. By this arrangement
dummy balancing pistons are not required, and the overall length
of the turbine is reduced.

A somewhat similar turbine has recently been constructed by
Messrs. Melms, Pfen^inger & Sankey, but in it the steam is

THE STEAM TURBINE. 189

admitted at one end, strikes an impulse wheel, and then passes
through blading of the Parsons type to the other end, where it
goes to the condenser. With a 500-kilowatt turbine constructed
on this principle, a consumption of 17*2 lbs. per kilowatt hour
has been recorded.

The Zoelly Turbine works on the same principle as the
Curtis ; but, like the Bateau, the shaft is horizontal. There are
usually ten rotary wheels having buckets round the periphery ;
these wheels are placed in two separate casings ; the shaft runs
right through the casings, and is supported by a bearing placed
between them. One casing is the high-pressure portion of the
turbine, the other the low-pressure portion. The steam, after
passing through the high-pressure portion, is conveyed by a pipe
to the low-pressure portion. If the reader will turn the illustra-
tion of the Curtis turbine up sideways, so that the shaft is
horizontal, and imagine that the dynamo shown is the high-
pressure portion of the turbine, he will have a good idea as to
the general appeM:'ance of the Zoelly turbine.

In this turbine the blades are made of nickel steel, and the
section decreases from the roots to the tips.

General Bemarks on the Steam Turbine.— It has already
been said that turbines of the Parsons type when coupled to
dynamos have given results, as regards steam consumption,
which have not been equalled by reciprocating engines. It
may, therefore, be well to look for the reasons which enable
such economy to be obtained. In the first place there is but
little initial condensation, as the steam after doing its work
passes away at the end of the turbine farthest from that at
which it was admitted. Condensation in the turbine can also
be reduced by superheating the steam to a high degree, and it
must be remembered that a turbine can be supplied with super-
heated steam with much less risk of injury than a reciprocating
engine, as in the former there are no rubbing surfaces in contact
with the steam. The bearings require to be oiled, but these
are outside the turbine proper, and the steam does not reach
them.

Another point which conduces to economy is this — owing
to the absence of internal friction, it is worth while to expand
the steam to a point much further than would be useful in
the case of a reciprocating engine. In a turbine, too, it is
possible to provide a very large opening through which the
steam can pass away to the condenser, as the reader will see by
referring to Fig. 69. It is found that a difference of even half
an inch in the vacuum makes an appreciable difference in the
consumption of steam. The reader will have seen from the

190 MECHANICAL ENGINEERING FOR BEGINNERS.

table given in the last chapter that the volume of steam
increases very rapidly as the vacuum approaches a perfect one;
in fact, the volume of steam with a 29-inch vacuum is double
that of steam with a 26-inch vacuum, so that with the high
vacuum we have double the volume of steam acting on the last
blades of the turbine. The saving of steam due to the last few
inches of vacuum is at the rate of between 4 and 5 per cent, per
inch of vacuum.

The economy of a steam turbine would probably be still
greater were it not for the necessity of having clearance
between the ends of the moving blades and the casing, and
between the ends of the fixed blades and the drum. Owing
to the somewhat great length of the turbine, the casing has a
tendency to " hog " under the temperature and pressure of the
steam. The drum, too, has a tendency to " whip " owing to its
length and high speed, and to the impossibility of making a
perfect balance of the blades. To insure that the blades will
not come in contact with the casing, it ia necessary to give them
a certain clearance, and this clearance allows a percentage of
steam to pass without doing any effective work. In small
turbines the clearance is proportionately greater than in
turbines of large power, and it is due to this fact that
turbines of less than 1,000 H.P. cannot at present compete
successfully on the score of economy with triple-expansion
reciprocating engines.

In spite of this clearance loss, the fact remains that turbines
of large size (as already stated) are more economical than
reciprocating engines. Apart from the economy attained with
turbo-electric generating plant, it has been found that the coal
used on the Channel steamers fitted with turbines is from
15 to 20 per cent, less than on those fitted with reciprocating
engines.

Other points in favour of the turbine for marine work are
these : — The weight of turbines and boilers for propelling a ship
is about 5 per cent, less than that of reciprocating engines and
boilers of the same power. In small vessels of the torpedo-
destroyer class, the whole of the turbine can be placed below
the water line, so that there is less risk of damage from gun fire
than is the case with a reciprocating engine. For passenger
steamers the absence of vibration is a great advantage. A good
feature of the turbine for both land and marine work is the
fact that no oil is required in the turbine itself ; the condensed
steam may therefore be pumped back into the boiler without the
intervention of oil filters.

Both for land and marine work the turbine has proved itself

THE STEAM TURBINE. 191

to be thoroughly reliable. An incident which was related to the
author a good many years ago by the engineer concerned may
not be considered out of place. Amongst the earlier turbines
made by Messrs. Parsons were some constructed for one of the
London electric light companies. These turbines were placed
on the first floor of the electric generating station, while the
ground floor was occupied by high-speed reciprocating engines.
When a serious fire broke out one night, the greatest eflforts
were made to keep up a supply of current, and the engineer
succeeded in keeping the turbine sets running after all the
reciprocating sets had failed owing to the heat and water.
The load which the turbine sets had to take up was enormous,
but by throwing buckets -of water on the dynamos to keep them
cool, the turbine sets came through the ordeal triumphantly.

The reliability of the turbine, together with the fact that it
may be over-loaded without serious risk and without a great
falling off in economy, are points very greatly in its favour.
In the case of electric generating stations, the saving effected
in the cost of dynamos, and of buildings and foundations, to say
nothing of the saving in oil and engine-room attendants, is very
marked indeed.

The limitations of the turbine are chiefly those due to its high
speed ; the speed is too high to enable it to drive by belts or
ropes without the intervention of gearing. If, however, the
electrical method of transmitting power is sidopted this objection
d o es n o t hold g ood. One rolling mill has recently adopted this
method of transmission; in this case steam turbines are used
for driving dynamos, and the rolls for rolling steel rails are
driven by motors. The arrangement is said to give entire satis-
faction. A similar arrangement has recently been introduced
into a cotton mill.

The steam turbine is somewhat handicapped by the fact that
it does not reverse. On board ship this difficulty is overcome by
having separate turbines for reversing. In marine work several
turbines are usually employed for driving the ship ; for instance
in the " Carmania," an Atlantic liner, one high-pressure turbine
drives a propeller amidships and two low-pressure turbines drive
the port and starboard propellers. The reversing turbines also
drive these shafts; when going ahead the reversing turbines
remain connected to the condenser, steam being, of course, shut
off

Another fact which has somewhat retarded the introduction
of the turbine is that it must condense if it is to complete
successfully with a reciprocating engine. In large towns water
must be paid for at a fairly high rate and this charge would be

192 MECHANICAL ENGINEERING FOR BEGINNERS.

prohibitive if the cooling water were used only once and then
thrown away ; but the tendency now is for generating stations
to be built on the outskirts of large towns where sufficient land
is available to admit of cooling towers being erected. The water
is cooled in these towers and is used over and over again.

On board ship the difficulty does not occur as sea water is
available for cooling purposes.

193

CHAPTER XL
ELECTRICAL CHAPTER.

It is almost essential at the present time that an engineer
should have some knowledge of electrical matters, and while no
attempt will be made to go at all deeply into the subject in the
present book, yet a few words dealing with the main facts of
practical work and with the relation of electrical units, such as
the volt, ampere, and ohm, to mechanical units of work, such as
horse-power, may be useful.

Production of the Electric Current. — In all cases where
current is required in any quantity or at any useful pressure
(electromotive force) it is produced by a dynamo or alternator,
and the principle upon which these machines work will now be
described.

In an ordinary horse-shoe magnet, such as may be purchased
in a toy shop, there are two poles, one north, the other south ; if
the magnet is straightened out as in a mariner's compass the
north pole (or north-seeking pole) will point to the north and
the south pole to the south. Surrounding each pole is a
magnetic field, and for the purpose of making calculations in
designing dynamos the strength or weakness of a magnetic field
is expressed by a number of imaginary magnetic lines of force
per square inch of the magnet face. When the two poles of a
magnet are brought together, as shown by Fig. 77, the lines of
force are supposed to flow from the north to the south pole and
constitute a magnetic field. The air is a bad conductor of these
lines of force, and if the poles are wide apart a weak field results.
Soft iron, on the other hand, is a very good conductor, and if a
portion of the space between the poles of a magnet is filled with
soft wrought iron, it assists the lines of force to flow, and the
space left unbridged has a stronger field than would be the case
without such partial bridging.

A horse-shoe magnet of the kind described is called a per-
manent magnet. A much more powerful magnet can be made
by passing an electric current round and round a bar of soft iron
or high permeability steel, and such electrically-excited magnets
are used in practice, excepting for the very smallest class of
dynamos, which are then usually known as magnetos.

194

MEOHAKIOAL ENGINEERING FOR BEGINNERS.

If a wire is placed between the legs of the magnet, as shown
by Fig. 77 (the black dot represents the wire in section), and is
moved sharply downwards so that it cuts the lines of magnetic
force, an electromotive force (E.M.F.) is induced in the wire,
and if the ends of the wire are joined so that the circuit is com-
pleted, a current of electricity flows round it. The E.M.F.,
which may be looked upon as electrical pressure, is spoken of as
so many volts ; the amount of current which will flow through
the wire (if the ends are joined) depends upon the E.M.F. and
the resistance of the wire.

With a permanent steel magnet and single wire the E.M.F.
induced would be too small to produce a serviceable current, and
obviously it would be very inconvenient to move the wire
rapidly up and down as shown. In actual practice an electrically-
excited magnet or number of magnets are used, and the wire, or

Fig. 77 and 77a. — Horse-shoe magnets.

wires, which have to cut the lines of force are placed on the
outside of a drum, as shown by Fig. 77a. The drum is made of
soft iron plates so as to facilitate the passage of the lines of
force ; by rotating the drum rapidly the wires on its outside pass
through a strong magnetic field, or, in other words, cut a large
number of lines of force flowing at right angles to the wires.
Such a drum with wires is called an armature, although an
armature need not necessarily take the form of a drum. It
should be mentioned here that in order to produce E.M.F. in a
wire the latter must pass from a weak field, or no field, into a
strong field or vice versd; or, if moved continuously in a field
of the same strength, the speed must be varied, otherwise no
E.M.F. will be obtained.

ELECTRICAL CHAPTER. 195

The amount of E.M.F. produced in a single wire on a drum,
when rotated in a magnetic field, depends upon the strength of
the field (or number of lines of force cut by the wire), and the
rapidity with which the wire is moved. Assuming that one wire
cuts sufficient lines of force at a speed sufficiently great to
produce an E.M.F. of 1 volt, then, if the drum is wound so
that there are 100 turns of wire round it, and it is rotated at the
same speed as before, an E.M.F. of 100 volts will be produced, .
as the voltage in each turn of the wire adds itself to the voltage
produced by the other turns. The wire must, of course, be
insulated and wound in the right way.

We have said that the amount of current which will flow in a

wire is dependent upon the E.M.F. and the resistance of the wire,

E
or, expressed as a formula, C = —• The amount of current is

JLv

expressed in amperes, and the resistance of a wire in ohms,* or
parts of an ohm, so that the formula, translated into actual

working terms, is amperes = — r Thus, assuming the E.M.F.

in a wire is 100 volts, and the resistance when the circuit is
<x)mpleted is 2 ohms, we shall have a current of 50 amperes
flowing. If, on the other hand, we have an E.M.F. of 1 volt and
a resistance of 2 ohms, we shall only have a current of half an
ampere flowing.

To return to the dynamo. If a single wire is used, as shown
in black by Fig. 77a, when the drum is rotated and the wire cuts
the lines of force by descending through them on the left-hand
side, the current will flow towards the spectator ; when the wire
cuts the lines of force on the right-hand side by rising through
them, the current will flow away from the spectator.* Thus the
current in the wire is reversed during every revolution of the
drum, and is called an alternating current. A dynamo arranged
with the wires or conductors arranged to give this effect is called
an alternating current generator, or alternator.

If, instead of a single wire on one side of the drum only,
the wire is prolonged and wound round the drum, as shown by
dotted lines in Fig. 77a, it will at once be seen that the current
flowing towards the spectator on the left-hand side and away
from him on the right-hand side will produce a continuous
current in the encircling wire, but the direction of the current
will be reversed once during every revolution. If, however, the
current flowing round the armature can always be tapped or

* Table xviii., giving the resistance of wires of various sections in ohmSi
will be found at the end of this chapter.

196 MECHANICAL ENGINEERING FOR BEGINNERS.

drawn off at the point where its E.M.F. is highest, and where the
current is flowing, say towards the spectator, and returned to the
armature at a point where the current is flowing away from the
spectator, a continuous or direct current will be obtained, and
will be available for use outside the dynamo. This tapping of
the current at the right place is effected by means of a com-
mutator. The commutator consists of a number of segments of
copper insulated from each other, and from the shaft ; to each one
of these segments one end of a wire, wound round the armature,
is connected, the other end being connected to the next segments
In actual practice the wire is taken a good many times round
the drum before its ends are connected to the commutator
segment. Brushes consisting of copper gauze pressed tightly
together, or of carbon, are used for collecting the current. One
brush or set of brushes is placed over, and pressed by a spring
upon, the segment of the commutator where the current is at the
greatest E.M.F., and where it is flowing in the right direction ;
another set of brushes returns the current to a segment at
the opposite of the commutator, thus completing the circuit. A
dynamo provided with such a commutator is called a continuous-
current or direct-current generator. If a large number of poles
are used, then a set of brushes is provided to collect the current
from the commutator at each pole.

We have seen that the volt is the unit of E.M.F., the ampere
the unit of current, while the ohm is the unit of resistance to
the passing of the current. One ampere of current at a pressure
of 1 volt is called 1 watt. Thus 5 amperes at a pressure of 2
volts =10 watts, or 2 amperes at a pressure of 5 volts are also
10 watts. A thousand watts are called 1 kilowatt, and dynamoa
are usually spoken of as giving so many kilowatts. Thus a
dynamo which gives an output of 100 amperes at 100 volts is a
10-kilowatt dynamo. A dynamo which gives 1,200 amperes at
100 volts is a 120-kilowatt dynamo ; while a dynamo which gives
3,000 amperes at 500 volts is a 1,500-kilowatt machine.

Electrical current cannot be produced without the expenditure
of energy, and it may be useful at this stage to see what relation
the electrical units bear to a mechanical horse-power. The rela-
tion is this — 746 watts = 1 electrical horse-power. An electrical
horse-power is, theoretically, the equivalent of 1 mechanical horse-
power, and if there were no losses in a dynamo, 1 brake horse-
power transmitted to it would produce 746 watts. But, as
a fact, there is, and must be, a certain loss of energy in every
dynamo. In the first place, there is the resistance of the wires
or coils carrying the current round the armature, and, in the
second place, there is the loss of that portion of the current

ELECTRICAL CHAPTER. 197

which is used for exciting the magnets. There are also the
losses due to mechanical friction and windage.

In a well-designed dynamo of, say, 200 kilowatts and upwards,
these losses may not exceed 6 per cent., so that for every 100
brake horse-power transmitted to the dynamo, the latter will
give an output of 94 E.H.P. ; in this case the efficiency of the
dynamo is said to be 94 per cent. In small dynamos such a
high efficiency is not obtained, while in very large dynamos the
efficiency may be greater.

A continuous-current dynamo can be used either for generating
electricity or it can be used for converting electrical energy back
into mechanical work ; when a dynamo is used for the latter
purpose, it is called a motor. The electrical losses in a motor are
the same as in a dynamo, but in a small motor of, say, 10 H.P.,
one would not expect an efficiency higher than 88 or 90 per cent.

It may be of interest to see how far an electrical horse-power
will go in the way of current for lighting purposes. A 16-candle-
power 200 to 250 volt incandescent (glow lamp) requires, when
new, about 60 watts per hour, so that 12|^ lamps will absorb
about 1 E.H.P (746 watts); 124 lamps will absorb 10 E.H.P. ;
and 1,244 lamps 100 E.H.P. The student must not, however,
fall into the mistake of thinking that a steam engine indicat-
ing 100 H.P. is capable of driving a dynamo when supplying
current for 1,244 lamps, as the losses already referred to must
be taken into account. In the first place, the loss due to
friction in the engine itself will probably reduce the available
power to 90 B.H.P. Then, assuming the efficiency of the dynamo
is 92 per cent., we shall have only 92 per cent, of 90 B.H.P., or
82-8 E.H.P., to dispose of. If another 5 per cent, (say 4*2 H.P.)
is lost through the resistance of the wires or leads conveying the
current to the lamps, we have only 78*6 E.H.P., or 58,635 watts,
to dispose of. Assuming each lamp requires 60 watts, then a
100 I.H.P. engine coupled to a dynamo will supply current for
977 lamps. In practice it is usually reckoned that 1 I.H.P. is
required for 8 to 10 16-candle-power incandescent lamps.

Arc lamps, or those in which the light is caused by the
current tearing away and making incandescent small particles
of carbon in jumping from one carbon to another, use less
current per candle-power than those of the incandescent form,
but the candle-power of an arc lamp is, of course, immensely
greater than that of a glow lamp. The E.M.F. required for
each arc lamp, if used singly, is about 50 volts, but five lamps
may be placed in series on a 240-volt circuit. The current used
is from 5 to 20 amperes, depending upon the size of the carbons
and candle-power of the lamps.

198 MECHANICAL ENGINEERING FOR BEGINNERS.

A 2,000 candle-power arc lamp of ordinary or open type
absorbs about 500 watts, so that in actual practice the engine
driving the generator must indicate nearly 1 horse-power for
every 2,000 candle-power arc lamp.

The carbons in an ordinary open type of arc lamp require to be
replaced after burning for 12 to 18 hours, but in the closed form
of are lamp, such as the Jandus, where the arc is enclosed and
is maintained in an atmosphere of carbon monoxide gas and
nitrogen, the carbons will last for 120 to 200 hours, depending
upon the size of the carbons and the amount of current sent
through them. From 6*5 to 7 amperes at 100 to 120 volts are
required with this form of lamp.

To return once more to the generator. In the earlier stages
of electric development, the two-pole horse-shoe form of dynamo
was considered the best for machines giving an output up to
200 kilowatts, but when larger machines than this were called
for, it was found that less material was required in dynamos
having a considerable number of poles, and multipolar machines
have now generally superseded bipolar machines even in the
smaller sizes.

Fig. 78 gives the end view of a 750-kilowatt multipolar
dynamo without its end bearing. In a multipolar dynamo, as
shown, the yoke and magnets are made of high permeability
cast steel — i.e., steel which presents very small resistance to the
lines of magnetic force. The poles being alternately north and
south, the lines of magnetic force do not require to travel right
across the armature as in a two-pole dynamo, but merely from
one north pole through a portion of the soft iron core of the
armature, and into the adjoining south pole.

In a direct-current generator giving current at a high voltage,
it is necessary to have a large number of segments in the com-
mutator in order to keep the difference of potential between
each segment as small as possible, and thus avoid any danger
of the current jumping across the insulation between the seg-
ments. If the commutator is of large diameter and the speed is
high, the centrifugal force of the segments is considerable. If,
on the other hand, the diameter of the commutator is small,
there is a risk of the current jumping from one set of brushes
to the next. Difficulties in connection with the commutators
of direct-current, high- voltage generators are not uncommon,
especially when such generators are coupled to steam turbines.

Direct current at a high E.M.F. can be obtained by working
two or more dynamos in series— i.e., the first dynamo supplies
current to the second, the second to the third, and so on. Thus
two dynamos, each giving 50 amperes at 3,000 volts, will, if

ELECTRICAL CHAPTER.

199

coupled up in series, give 50 amperes at 6,000 volts, and three
such machines 50 amperes at 9,000 volts. This system was first
carried out on a practical scale by M. Thury, and is known as
the Thury system.

The reason why high voltages are required is this — a current
of 1,000 amperes at a pressure of 10 volts has the same energy
as a current of 10 amperes at a pressure of 1,000 volts, but the
former current requires a conductor having about i of a square
inch sectional area to carry it, while a current of 10 amperes

Y, Yoke or magnet ring.
B, Part of brush ring carrying
three brushes.

C, Commutator.
S, Shaft.

Fig. 78. — Twelve-pole direct-current dynamo.

requires a conductor having only ^-^ part of a square inch to
carry it, and as copper is very expensive, it is necessary, when
current has to be transmitted to a great distance, to transmit it
at as high voltage and small amperage as possible.

An alternator is much more suitable for producing current at
a high E.M.F. than a direct-current generator, as it requires no
commutator. An alternating current can be used for lighting

200 MECHANICAL ENGINEERING FOB BEGINNERS.

purposes, provided the alternations are sufficiently rapid — viz.,
from 40 to 50 per second ; an alternating current also has the
following valuable property: — If two conductors are placed
side by side, and an alternating current is sent through one
of them, it will induce a current (having alternations in the
opposite direction) in the other conductx)r, and the E.M.F. of
the secondary or induced current can be made either higher
or lower than that of the primary current. Thus, if a wire
of large diameter, carrying a current of a good many amperes,
is placed alongside a smaller wire, and having a greater
resistance, the current induced in the latter will be of higher
voltage, but of smaller amperage, than in the primary wire,
and vice verad.

By the aid of a suitable transformer, an alternating current
can therefore be transformed either to a higher or lower E.M.F.
A continuous or direct current will only induce a current in a
second wire at starting and stopping. In lighting a town having
a number of outlying districts, it is customary to generate an
alternating current at a high E.M.F., and to transmit at this
high tension to a number of transformer stations, where the
current is transformed down to the E.M.F. at which it will be
used on the consumer's premises. The Board of Trade will not
allow the E.M.F. of any current entering a private house or shop
to exceed 500 volts. As a rule, the E.M.F. of the current used
in a private house or shop does not exceed 220 or 240 volts.

If a current has to be transmitted to a distance of many miles,
and the voltage decided upon is too high for the insulation of
the alternator, the current can be generated at a moderate
E.M.F. and then transformed up (by a step-up toansformer),
transmitted to its destination, and then transformed down
again.

The insulation of conductors carrying current at a very high
E.M.F. can be more easily arranged in a stationary transformer
than in the generator itself, but there is a loss every time the
current is transformed up or down.

Alternators, Single and Multi-phase. — In a large alter-
nator, where, as we have seen, it is not necessary to have a
commutator, it is customary to rotate the field magnets, and
to have a fixed armature. A large number of poles, alternately
north and south, are arranged on the rotating magnet, while the
armature is outside it. Fig. 79 shows the arrangement diagram-
matically with'a uni-coil winding — {.e.j one slot or coil per pole
piece. Plan 1 shows the direction of the current in one of the
armature coils while a north pole is passing under the right-
hand portion of the coil, while Plan 2 shows the reversal of the

ELECTRICAL CHAPTER.

201

current while a south pole is passing under the left-hand
portion of the coil.

ILllIB

Planl

[BZ.11

Plan 2.

Fig. 79.— Alternator,

Two- and Three-phase Alternators. — An ordinary single-
phase alternating current has one great disadvantage — viz., it
will not start an electric motor. This difficulty in connection

202

MECHANICAL ENGINEERING FOR BEGINNERS.

with the alternating current has, however, been overcome by
employing three-phase alternators and motors. The meaning of

Fig. 82.

Fig. 83.
Alternating current diagrams.

a three-phase alternating current can best be explained by the
help of diagrams.

ELECTRICAL CHAPTER.

203

Fig. 80 represents diagrammatically a simple form of single-phase
alternator having a north and south pole, and a single conductor
on the drum, and Fig. 81 represents graphically the alternating
current produced by one revolution of the drum carrying the

Fig. 84. — Alternate current diagram.

conductor; the highest point of the curve represents the greatest
E.M.F. of the current, and the lowest point the zero. One
revolution of the drum produces one complete period, so that, if
the drum revolves 50 times in a second, the periodicity of the
alternator (having one pair of poles) will be 50 complete periods

204

MEOHANIOAL ENGINEERING FOR BEGINNERS.

per second. If a second conductor is placed on the drum the
current produced will be as shown by Fig. 82, and will be a
two-phase current. If the number of revolutions of the drum
remains the same as before, the periodicity of the two-phase
current will also remain unchanged — viz., 50 per second. If a
third conductor is placed on the drum the resulting current will
be a three-phase current, and will be as shown by Fig. 83.

Instead of two poles, as shown, there are, as previously
mentioned, usually a large number of poles; these poles are
rotated while the armature is stationary. The arrangement of a
uni-coil three-phase alternator is shown by Fig. 84. In actual

Fig. 85. — Alternating current generator.

practice, however, the two legs of the U-coil are seldom separated
so widely as to come one over a North pole and one over a
South pole. The winding of a three-phase alternator varies
greatly, but Fig. 85 is a good example of such winding.

Periodicity of Alternating Currents. — The periodicity of
an alternating current, if the number of poles and number of
revolutions of the alternator are known, may be found thus : —

P = N X R;

where P = periodicity per second.

N = number of pairs of poles.
R = revolutions per second.

ELECTRICAL CHAPTER. 205

If it is desired to find the number of revolutions per minute
an engine or turbine must make to give a certain periodicity per
second, the formula is —

where R = revolutions per minute.
P = periodicity per second.
N = number of pairs of poles.

JS'^icamp^c.— Suppose an alternator, which has to be driven by a turbine,
has 4 poles (2 pairs), and the periodicity required is 50 per second, at what
speed must the turbine nin ? The calculation is —

— - — = 1,600 revs, per minute.

Or, suppose an alternator, requiring to be driven by a high-speed
engine, has 20 poles (10 pairs), and the periodicity required is 50 per
second, at what speed must the engine run ?

50 X 60 ^^

— j^r — = 300 revB. per minute.

Before leaving the subject of generators, it may be well to
explain the difference between series-wound, shunt-wound, and
compound-wound dynamos. A series-wound dynamo is one in
which the whole of the current generated in the armature is
passed round the magnets to excite them. Such a dynamo is
seldom met with, as, if extra resistance is put into the main
circuit through which the current is flowing, the current will be
reduced, and the exciting current will also be reduced ; the same
thing happens if the speed of the dynamo falls off. In both
these cases the exciting current is weakened just when it should
really be strengthened. Series- wound motors are, however, used,
as a strong field is required at starting.

A shunt-wound dynamo is one in which the current is split up
when it leaves the collecting brushes ; a portion of the current
is sent round the magnets, and the remainder flows into the main
circuit. If extra resistance is put into the main circuit, the
current finds an easier path round by the field magnets, and so
strengthens the field. The amount of current flowing round the
field magnets is usually regulated by means of a resistance and
switch; thus the strength of the field can be increased or
decreased at will.

A compound-wound dynamo is one in which a few turns of
series winding are placed over the shunt winding, and arranged
to act in opposition to it. Thus, if the speed of the generator
should fall and the main current be reduced, the opposing power

206 MECHANICAL ENGINEERING FOR BEGINNERS.

of the series winding will also be reduced, and the shunt wind-
ing, freed from this opposition, will more strongly excite the
magnets.

A separately-excited dynamo is one in which the exciting
current is supplied from another dynamo, or from some inde-
pendent source. All alternators require to be separately
excited.

Rotary Converter and Motor Q-enerator. — If alternating
current is employed for transmitting energy to a distance, and
it is required to convert the alternating current into continuous
or direct current, this can be done in two ways. The first is
by means of a rotary converter. This consists of a motor, the
armature of which is driven by the alternating current ; over the
winding required* for this alternating current is placed winding
suitable for producing continuous current. When the armature
is rotated, the latter winding produces direct current in the same
way as in a direct-current generator.

A motor generator consists of an alternating-current motor
coupled to and driving a continuous-current generator. A motor
generator is practically the same thing as a rotary transformer,
only the alternating-current motor and the direct-current gen-
erator are two distinct machines coupled together, instead of
being combined, as in the case of the rotary transformer. It is
found that better regulation of the E.M.F. can be obtained by
the use of two separate machines.

Primary Batteries and Accumulators. — A small current
at a low E.M.F. can be obtained from primary batteries or cells,
but these, although useful for electric bells and telephone work,
hardly concern the mechanical engineer, as the production of
current on a large scale by batteries is out of the question on
account of the cost.

The simplest form of cell, known as a Daniell cell, consists of a
plate of zinc and one of copper immersed in dilute sulphuric acid;
the acid eats away the zinc, and, if the zinc and copper plates are
connected by a wire, a current flows from one plate to the other
through the acid and through the wire. The E.M.F. of such a
cell is 1 '05 volts. In the Leclanch^ cell, largely used for electric
bells, there is a rod of zinc, also a porous pot containing a block
of carbon, small pieces of crushed carbon, and black oxide of
manganese; these replace the copper plate of the Daniell cell.
The zinc and porous pot are immersed in a solution of sal-
ammoniac. The E.M.F. of this cell is 1*4 volts.

Accumulators. — Accumulators are used for storing electricity,
if such an expression may be used, for the action is really a
chemical one. An accumulator consists of two or more plates,

ELECTRICAL CHAPTER. 207

positive and negative, pierced by a large number of holes. The
holes in the negative plate are usually filled with pellets of lead
oxide, while the holes in the positive plates are filled with pellets
of peroxide of lead ; these plates are immersed in dilute sulphuric
acid ; a current is passed from one plate to the other through the
acid, when a certain chemical change takes place. After the
current has passed for a sufficient length of time, the accumulator
is said to be charged. That is to say, if the plates are connected
together by a wire, a current will flow in the direction opposite
to that in which the current entered the accumulator. Each cell
is charged until the E.M.F. reaches about 2*5 or 2*6 volts ; when
the charging wires are disconnected, the E.M.F. falls to about
2-1 volts, and as the accumulator discharges its current the
E.M.F. gradually falls to 1*85 volts, when the accumulator is
practically discharged. Any further discharge is injurious to
the accumulator.

The capacity of an accumulator in amperes depends upon the
surface of the plate or plates. In practice there are several
plates in each cell ; all the positive plates are connected to one
bar, and all the negative plates to another bar. To obtain a
higher E.M.F. than 2*1 volts, a number of accumulators may
be placed in series ; in this case the positive terminal of one
accumulator is connected to the negative terminal of the next,
and so on. Thus, if two accumulators are placed in series,
the E.M.F., when charged, is 4*2 volts* ; if 10 accumulators are
placed in series, the E.M.F. is 21 volts; and if 50 are in series,
the E.M.F. is 105 volts. If it is essential to maintain the total
E.M.F. constant while the accumulators are being discharged, a
few additional cells must be added to the series to compensate
for the falling off of E.M.F.

In order to charge accumulators, direct current must of course
be used, and the E.M.F. of the current must be a little higher
than that of the whole of the cells being charged in series.
Accumulators may be coupled up in parallel ; this means that
the whole of the positive plates are connected together, and the
whole of the negative plates together. If coupled up in this
way, the total E.M.F. is only that of a single cell, but the total
current in amperes is increased in proportion to the number of
cells employed.

The capacity of an accumulator is known by ampere-hours;
thus a 24 ampere-hour accumulator will give 1 ampere for
twenty-four hours, or 2 amperes for twelve hours, or 4
amperes for six hours. The rate of charging and discharging

*The 4- volt accumulators used for motor-car ignition are really two
2*1 cells coupled in series in one case.

208 MEOUANICAL ENGINBERING FOB BEGINNERS.

is stated by the makers, and should not be greatly exceeded ;
if it is, gas is formed too quickly, and tends to dislodge the
pellets.

The acid used consists of 8 parts of sulphuric acid to 5
parts of water, and has a density of 1*2. As the accumulator
discharges the acid becomes weaker, so that by placing a hydro-
meter in the acid and noting how far it sinks, the amount of
charge in the accumulator may be known approximately.

Accumulators are very useful for country house lighting, as
they can be charged in the day time, and the current used
during the evening and night time. They are also useful in
small central stations as a reserve, and to maintain a supply of
current during the night, should the demand not be sufficient to
warrant the running of an engine and dynamo. Accumulators
are also used sometimes as a steadier for the current ; in this
way, — the engine and dynamo supply the main demand for cur-
rent, any excess current going through the accumulators ;
should the demand be suddenly greater than the engine and
dynamo are capable of meeting and the E.M.F. falls, then the
accumulators automatically discharge their current, and so
assist the engine and dynamo. When the demand falls off
and the E.M.F. rises, some of the current from the dynamo
flows round by the accumulators and recharges them.

The efficiency of an accumulator is from 70 to 80 per cent. — »
that is to say, it will return 70 or 80 per cent, of the current put
into it.

The disadvantages of accumulators are — (1) Their first cost.
This is somewhat high, as for an E.M.F. of 120 volts about 70
accumulators are required, as one can only reckon upon the
E.M.F. of the cells when nearly discharged — viz., 1*85 volts;
(2) the cost of upkeep, which is rather high ; and (3) their great
weight. Objections 2 and 3 prevent accumulators coming into
general use for traction purposes.

The instruments used for measuring the electric current are
the voltmeter, for measuring the E.M.F. ; the ammeter (or
ampere meter), for measuring the amount of current flowing ;
and the wattmeter ; the latter measures the volts multiplied by
the amperes. An automatic cut-out is an instrument for
automatically breaking the circuit should an excessive current
pass.

A summary of the electrical units, with which it is desir-
able that the mechanical engineer should be familiar, is as
follows : —

ELECTRICAL CHAPTER.

209

Ampere
Volt

Ohm

- unit of current.

= unit of pressure or e

force (E.M.F.).
= unit of resistance.

1 ampere x
746 watts
1,000 watts
1 megohm

1 volt = 1 watt.

= 1 electrical horse-power.

= 1 kilowatt.

= one million ohms.

Volts

Volts ,
amDeres. -. ■- ohm

electromotive

Ohms ^ Amperes

Amperes x ohms = volts.

The following table giving the resistances in ohms per 1,000
yards of copper wires of different diameters, and the amount of
current in amperes carried, may possibly be useful. The current
given is based upon 4,000 amperes per square inch of sectional
area. The current carried by a cable having several strands
can also be ascertained by the table — thus, a ^\ cable will carry
A current three times greater than a single No. 20 wire, or
12 amperes. A /^ cable will carry 28 amperes, &c. : —

TABLE XVIII.

Working

Current

8.W.G.

Diameter.

Sectional Area.

in 1,000

at 4,000

Yards.

Amperes
per Sq. In.

Inch.

Mm.

Inch.

Mm.

Ohms.

Amperes.

•028

•711

•0006

•397

38-46

2*4

•036

•914

•0010

•657

23-26

4^0

•048

122

•0018

1167

1310

7-2

•064

1-62

•0032

2075

7-36

12-8

•072

1-83

•0040

2627

6-81

16-0

•080

203

•0050

3-243

4-71

200

•092

2-34

•0066

4-287

3-57

26 6

•104

2-64

•0085

5 48

2-78

34-0

•116

2-95

•0106

6-818

2-24

42-4

•128

3-25

•0129

8-302

1-84

61-5

•144

3-66

•0163

10-507

1^45

65-2

•160

406

•0201

12'972

1^18

80-0

•176

4-47

•0243

15-695

•99

97-2

•192

4-88

•0289

18-679

•83

1156

211

CHAPTER XII.
HYDRAULIC MACHINERY.

The words "hydraulic machinery'' cover a somewhat wide field :
we will first consider hydraulic machines, such as presses,
riveters, and lifts, and then pass on to water-wheels, turbines,
and pumps.

Power can be transmitted to considerable distances by means
of. water under pressure in a safe, clean, and fairly economical
manner, and such transmission is usually employed in cases
where large powers are required to be exerted during a short
space of time and intermittently, as in hydraulic riveters,
hydraulic presses for flanging boiler plates, <fec. Hydraulic
power is also largely used for cranes and lifts where the power,
although not necessarily great, is required intermittently.

The principles governing the construction of hydraulic
machinery in which water under high pressure is employed
are fairly simple, none of the problems arising in connection
with the use of steam, such as initial condensation, re-evapo-
ration, (fee, are met with, and if the amount of power required
is known it is not a difficult matter to design a machme to
give it.

The hydraulic press fitted with a small hand pump, illustrated
by Fig. 86, shows the principle upon which most hydraulic
presses, riveters, and lifts work. The action is as follows : —
The ram of the press is of large diameter while the pump
plunger is of very small diameter. The small plunger on being
raised draws in water through the valve at the bottom of the
pump ; on being pressed down the plunger forces water through
the valve at the side of the pump into the .main cylinder which
contains the ram of the press. Now, if the area of the ram is
iifty times greater than that of the small plunger, every 1-lb.
pressure exerted on the plunger will give a pressure of 50
lbs. on the ram : if a pressure of 100 lbs. is exerted on the
small plunger a pressure of 5,000 lbs. will be given to the
large ram.

Of course the large ram moves very slowly if the water is
forced into the cylinder by a very small plunger, and in practice
such a hand pump would not be used, but even with pumps

212

MECHANICAL ENOINEBKINO FOR BEGINNERS.

driven by steam power the motion of the ram would be too
slow if the water were pumped directly to the cylinder. This
difficulty is overcome by pumping the water into an accumulator,
storing it there under pressure, and drawing from the accumu-
lator as required.

y/////////////^

Fig. 86. — Hydraulic press.

A well-designed hydraulic accumulator having a ram 8 inches
in diameter and a stroke of 14 feet is shown by Fig. 89.
Drawings of accumulators are not frequently given in text

Fig. 87. — Portable hydraulic riveter.

Fig. 88.

books or technical journals, and the reader would do well to
examine the illustration carefully, comparing it with the small
drawing of an accumulator which is taken from a work of a

HYDRAULIC MACHINERY.

213

popular character. The accumulator illustrated on the larger
scale shows the simplicity and directness characteristic of all the

mw/.

Fig. 89. — Hydraulic accumulator.

designs of the late Mr. Tweddell (and of Messrs. Fielding k
Piatt). It will be seen how, by inverting the cylinder, the
gland has been made accessible for packing, how the weight

214 MECHANICAL ENGINEERING FOR BEGINNERS.

in the casing is C5amed by the cylinder without the necessity
for supporting it from a cross beam at the top, and how the
cylinder walls form the inside of the casing, enabling a single
outer casing to take the place of the inner and outer casings
necessitated by a fixed cylinder and moving ram. In very large
accumulators a supporting top cross girder is sometimes used,
but the moving cylinder is, of course, retained. To proceed
with the description: — The accumulator consists of a cylinder
which is free to move up and down on a ram; the cylinder carries
a large wrought-iron casing which is filled with scrap iron, slag,
or any material sufficiently heavy to give the required weight
and pressure per square inch on the water. When water under
pressure is pumped into the accumulator the cylinder carrying
the casing rises, and the water in the accumulator remains under
pressure until it is required. When water is taken from the
accumulator the cylinder and casing descend for a short distance,
and a further supply of water is pumped in, and, as the capacity
of the accumulator is much greater than that of the cylinder of
any one hydraulic machine, there is always available a supply of
water under pressure.

In practice steam-driven pumps are usually employed to force
the water into the accumulator, and there is a simple device by
means of which the pumps are stopped when the accumulator
has reached the top of its stroke, and automatically started when
the accumulator has Tallen a certain distance.

The pressures used in hydraulic engineering vary from 750
lbs. per square inch to 2,240 lbs. per square indb. The pressure
used by Mr. Tweddell, and still employed by Messrs. Fielding
& Piatt, in connection with hydraulic riveters, is 1,500 lbs.
per square inch.

The credit for the introduction of hydraulic riveting machinery
is due to the late Mr. R. H. Tweddell, and to him alone. This
gentleman had considerable difficulty in getting any firm of
engineers to take up and manufacture his plant. Shipbuilders,
boilermakers, and others considered that it was impossible
to make a riveter sufficiently portable to be of much
use, and they doubted whether the cost of closing rivets by
hydraulic power would be as low as was the case with hj:nd-
nveting. However, after Messrs. Fielding <fe Piatt undertook
the manufacture of the riveters, and their advantages became
known, hydraulic riveting soon became general, and, at the
present time, probably no boiler works or shipbuilding yard is
without a hydraulic riveting plant. In addition to doing the
work more quickly and cheaply than by hand, the work done is
sounder and better. The comparison of a section cut through

HYDRAULIC MACHINERY.

215

af riveted Joint, the rivets of which have been hydraulically
closed, with a section cut through a pair of plates and through

Fig. 90. —Hydraulic riveter.

rivets which have been closed by hand, shows the difference
at once.

216 MECHANICAL BNOINEBRING FOB BEOINNBR8.

Fig. 90 shows a large fixed riveter and plate closer. Water
is first admitted to the cylinder A, and moves forward the plate
closing cup C (shown to a larger scale at the bottom of the
illustration); this cup presses the plates firmly together, and
holds them so while pressure is admitted to cylinder D; the
effect is to move forward the die H, and to close the rivet.

In the earlier days of hydraulic riveters water under full
pressure was used for moving the rams through the whole of the
outward stroke, the small ram E being used for the return
stroke only. This was a somewhat wasteful way of using the
water, and modem riveters are either supplied with water under
two pressures, the lower pressure being used to bring the die up
to its work, and the full pressure for closing the rivet; or the
small ram is used for bringing the die up to its work, water
being permitted to flow into the cylinder from a tank 20 to 30
feet overhead, and the full pressure being used for doing the
actual work.

In cases where it is inconvenient to bring the work, such
as a long lattice girder, to the riveter, portable riveters are
employed. A portable riveter is shown by Fig. 87. It
will be noticed that the cylinder is placed at the end of the
riveter farthest from the dies; this arrangement admits of
the dies being used in confined spaces. The riveter can swivel
at A and B, also at the hook C by which it is hung from the
crane chain, so that the dies may be turned into any desired
position. Water is brought by the pipe D ; it is carried to the
central gudgeon pin A, and from thence, at the back of the
riveter to the cylinder. The pipe leading from the crane to D
consists of a copper pipe arranged spirally round the chain; when
the riveter is lowered the spiral extends ; when it is raised the
spiral closes ; so that the water supply is not affected by raising
or lowering the riveter.

The joint between the moving piston or ram, and the cylinder
walls is made by a leather U-ring, an enlarged view of which is
given in Fig. 88; with this form of joint the greater the pressure
of water, the more the side of the U -leather is forced against
the cylinder walls. A small ram placed at the back of the
riveter, used for opening the jaws, is not shown. The riveter
arms are made of cast steel ; the cylinder is lined with bronze.

It will be noticed that the cylinder must be curved at a
radius struck from the gudgeon pin A, in order that the riveter
arms may move about this centre. If the reader will ask any of
his engineering friends how such a cylinder is bored, two out
of three will probably say that it can only be done by special
machinery, and it is so stated in a well-known text-book. This,

HYDRAULIC MACHINERY. 217

however, is not correct; the curved cylinder at the end of a
riveter arm can be truly bored in an ordinary lathe, and,
although this book does not deal with mechanical processes an
exception will be made in the present case. The method of
boring which the author saw in use some years ago is as
follows : — The boring tool is attached to the lathe face plate, an
upright pillar of the size of the gudgeon pin A is bolted to the
saddle of the lathe, the riveter arm is carried on this pillar
horizontally, with its cylinder towards the tool. The riveter
arm is free to turn on the pillar, and the latter is placed at such
a distance from the centre of the lathe that the tool as it
revolves will take a cut out of the cylinder if the latter is
pressed forward. A piece of hard wood, about a couple of feet
long, is placed at the back of the cylinder, and carried to the
back centre of the lathe. The lathe is set to work, the tool
revolves, and the turner feeds the cylinder forward by means of
the hand- wheel on the back centre; the arm carrying the cylinder
being pivoted on the pillar referred to, the cylinder is bored to
the right radius.

Hydraulio Flanging Press. — When a boiler plate to be
flanged is not of too great a diameter it is done in a press similar
to that shown by Fig. 86, but a powerful press may have two or
three rams and cylinders in place of the single ram shown. The
plate is made red hot and is placed between dies, as shown by
dotted lines ; the plate is flanged at one operation by the rising
of the ram.

In cases where the plate to be flanged is of irregular shape
and cannot well be done in the manner described, the flanging
is done a piece at a time by means of a press, as shown by
Fig. 91. One ram holds the plate, while the second ram turns
it over ; sometimes there is a third ram placed horizontally so
that it may square up the flange which has been turned over.
In working with these presses it is necessary to see that the
plate is not allowed to get too cold while the operations are
being carried on, as it has been found that steel worked at a blue
heat loses a large proportion of its strength.

Fig. 92 shows a hydraulic punching machine; this hardly
needs a description as its action is the same as that of the fixed
riveter, but without the plate-closing arrangement. Hydraulic
shearing machines working upon the same principle are also
used in boiler works and ship yards.

Hydraulio Lifts and Cranes. — There are two methods of
raising the cage of a hydraulic lift. The first is to place it
directly on a ram, such as is shown in the illustration of the
hydraulic press. This method is simple and safe, but necessitates a

218

MECHANICAL ENGINEERING FOB BEGINNERS.

ram and cylinder of great length ; it also necessitates a very deep
pit to receive the cylinder. The weight, too, of such a ram is
somewhat great and a considerable amount of power is expended
in raising it ; the weight of the ram can, of course, be balanced
by counter weights and chains, but these introduce an element of
risk and are very undesirable. A hydraulic balancer somewhat
similar to an accumulator is sometimes used, into which some of
the water is returned on the down stroke of the ram.

The second method of raising the cage of a hydraulic lift and
one which is usually employed in hydraulic cranes, is by means
of a jigger or multiplier as shown by Fig. 93. By the use of a
jigger a weight can be lifted through a considerable distance with
a comparatively short ram and cylinder. The chain is passed
over multiplying sheaves, and every sheave on the ram multiplies

Fig. 91.

-Hydraulic flanging
machine.

Fig. 92. — Hydraulic punching
machine.

the motion by two ; the reason for this will be seen by glancing
at the illustration, as for every foot by which the ram rises there
must be a foot of chain on each side of the sheave. Thus if there
are three sheaves and the ram rises a foot, a motion of 6 feet will
be given to a weight at the end of a chain. A certain amount
of power is lost by the friction of the chain and pulleys, but the
convenience of this method of multiplying the lift outweighs the
disadvantage. Rams arranged in this way — viz., with the chain
passing over sheaves — are used for slewing or turning cranes, as
well as for raising the weight.

Hydraulio Jack. — A hydraulic jack, as shown by Fig. 94, is
very useful for raising heavy weights when these cannot be dealt
witn by a crane. The principle upon which the jack works is
the same as that of the press shown by Fig. 86, but the jack is

HYDRAULIC MACHINERY.

219

self-contained ; the pump is placed inside the upper portion of
the casing; this portion also holds the water which the pump
plunger forces down into the space A, and causes the upper
portion of the jack to rise. The weight to be raised is placed
either on the top of the jack or on the projecting piece B. It
was by the aid of such jacks that the steamer "Great Eastern" was
successfully launched after having resisted all previous attempts

I 1

M- -'

Fig. 93. — Hydraulic jigger.

Fig. 94. — Hydraulic jack.

to move her. The Menai tubular bridge, too, was raised into
position, a few inches at a time, by means of hydraulic jacks.

Hydraulic Tests. — Parts of machinery, such as steiam cylin-
ders, valves, boilers, hydraulic cylinders, &c., which are required
to withstand pressure, are usually tested hydraulically, a hand
pump such as is shown in Fig. 86 being used. In such tests it
is necessary that the highest part of the piece under test should
be provided with an air cock, and the whole of the air be ex-
pelled before the full pressure is applied. If this is not done.

MECHANICAL ENOINEBBIKQ FOR BEGINNERS.

and the piece under test fails to bear the requisite pressure, the
compressed air may cause broken pieces to fly in every direction,
possibly with serious results. Water, unlike air, is practically
incompressible, and in the event of the failure of the piece under
hydraulic test, a loud noise is heard, the water escapes, but no
damage is done, provided no air is present

Water Wheels and Turbines. — In the days before steam
power was available, mills, in which mechanical power was
required, were built on the banks of streams and rivers, in
places where a fall of water could be obtained. If the natural
fall was not sufficiently great, it was sometimes increased by
placing a dam across the stream, and thus raising the level of
the water on one side of the dam. The water at the higher level
then had potential energy, or energy of position, and this energy
was converted into mechanical work by means of water wheels.

Water wheels are seldom constructed now, as the capital which
has to be expended in constructing the necessary masonry work
and wheel is usually greater than that needed to purchase a
small engine and boiler capable of giving as much power as the
cumbrous wheel, and the interest on the capital saved may go
some way towards providing coal for the steam plant.

In cases where an adequate supply of water with a good fall
is available, and considerable power is required, water turbines
are employed in preference to water wheels ; they are much less
cumbrous, require less masonry, are more efficient, and can be
governed more accurately than the latter. The leading features
of water wheels will, therefore, only be described briefly before
passing on to turbines.

Water wheels are of three kinds — viz., Undershot, Overshot,
and Breast wheels. Undershot wheels are used in cases where
the fall is low, say, 1 to 2 feet, and where a fair amount of water
is running to waste. As its name implies, the undershot wheel
is one in which the water strikes the vanes of the wheel at a
point below the axis, and causes the wheel to rotate. The
efficiency of an ordinary undershot wheel is very low — viz., from
25 to 30 per cent. — ^but if the buckets are curved, as in the
" Poncelet '** wheel, a much higher efficiency is obtained. The
Poncelet undershot wheel partakes somewhat of the nature of a
turbine.

Overshot wheels are employed in cases where the fall is
sufficiently great, 15 feet or more, to allow of the water passing
over the top of the wheel and descending on the far side. In
this type of wheel the vanes are shaped like buckets, so that the
weight of the water, as well as the energy with which it enters
the buckets, is utilised for turning the wheel. An efficiency of

HYDRAULIC MACHINERY. 221

from 60 to 75 per cent, can be obtained with this form of
wheel.

Breast wheels are used in cases where the fall, 5 to 15 feet, is
hardly sufficient to enable an overshot wheel to be used, and where
the velocity of the water is not great. In the breast wheel the
water is delivered just above the centre line or axis of the wheel,
and its weight only is used for turning the wheel. The efficiency
of an ordinary breast wheel is from 30 to 50 per cent.

If the amount of water flowing per minute, the distance
through which it falls, and the efficiency of the water wheel are
known, it is an easy matter to calculate the amount of power
which can be obtained. An example is given in connection with
turbines later.

Turbines. — ^The turbines chiefly used in Europe and in the
United States of America are of the reaction type, and the
meaning of the words reaction turbine is as follows : — We have
read that, according to Newton's first law of motion, "every
body continues in a state of rest or of uniform motion in a
straight line, except so far as it is compelled by force to change
that state." Now, if a body of water is moving in a certain
direction, and is compelled to change that direction, force is
required to effect the change ; in the case of a reaction turbine,
the vanes of the moving wheel are arranged (see Fig. 95) so as to
compel the water to change its direction, and the resistance
offered by the water to this change of direction exerts a re-
actionary force on the vanes of the moving turbine wheel ; it is
this reactionary force which drives the wheel.

In the impulse form of turbine, the buckets of the wheel are
arranged, not so much with a view to change the direction of the
jet of water, as to oppose its progress and bring it to a state of
rest. The energy which the water has acquired in falling from
a height — viz., kinetic energy, or energy of motion, is given up
to the buckets, and is utilised for turning the wheel.

Parallel Plow or Axial Turbine. — Fig. 95 shows diagram-
matically a turbine of this type, in which the water flows in a
direction parallel with the axis of the turbine. This form of
turbine is generally known as the Jonval, from the name of the
engineer who introduced it. The illustration is almost self-
explanatory ; the water descends through the fixed guide vanes,
which give it a certain direction ; the moving vanes prevent the
water from continuing in its proper course and deflect it ; the
reactionary force imparted to the vanes causes the wheel to
rotate. This form of turbine is usually governed by throttling
the water as it leaves the suction tube A.

Badial Inward Plow Turbine. — Fig. 96 shows a turbine

222

MECHANICAL ENGINEERING FOR BEGINNERS.

of this type, in which the water flows into the turbine wheel
in a direction at right angles to the axis of the wheel. This
form of turbine was introduced by Lord Kelvin.

Badial Outward Flow Turbine. — Fig. 97 shows a turbine
of this type, in which the water flows in a direction radial to the
axis, but outwardly.

Fig. 95.— Parallel flow or axial turbine (Jonval type).

In addition to the three reaction turbines illustrated, there is
the Mixed flow turbine, which is practically a combination of
those shown by Figs. 95 and 96. In this turbine the moving
blades, instead of ending as shown by Fig. 96, are prolonged and

HYDRAULIC MACHINERY.

223

arranged so that they compel the inflowing water to change ita
course from a horizontal one to a vertical one.

Some of the advantages and disadvantages of each type of
reaction turbine will now be considered.

f /xg^

96. — Radial inward flow turbine.

The outward flow turbine, shown by Fig. 97, was practically
the earliest form of reaction turbine, and was introduced by a
French engineer named Fourneyron. The chief disadvantage of
this turbine is the difficulty of governing it with accuracy, and a.

224

MECHANICAL ENGINEERING FOR BEGINNERS.

further disadvantage is that it must be placed at the lowest
point of the fall, near to the tail race. Turbines of this type

Fig. 97. — Radial outward flow turbine.

were employed in the first portion of what is probably the
largest turbine installation in the world — viz., the Niagara Falls
installation. The second instalment of turbines at this gene-

HYDRAULIC MACHINERY. 225

rating station were, however, of the inward flow type. This
installation will be referred to again later. The efficiency of
the outward flow reaction turbine is from 75 to 80 per cent.

The inward flow turbine has two advantages over that of the
outward flow type, viz. : — (1) Its variation of speed can be
controlled with greater accuracy, and (2/ it may be used with
a suction tube. The use of this tube enables the turbine to be
placed (within certain limits given later) near the upper level
of the water, the fall taking place in the suction tube. The
efficiency of this turbine is from 75 to 80 per cent.

The parallel flow or Jonval turbine can also be governed with
fair accuracy ; it may be used with a suction tube, and gives a
slightly higher efficiency than turbines of the radial flow type.
The efficiency of a good Jonval turbine is from 80 to 85 per cent.
The Jonval turbine, too, can be readily constructed for working
in cases where the head of water varies. In such cases the
turbine is provided with two or three rows of vanes, arranged
concentrically, instead of a single row. When a good head of
water is available the inner row is used, and the outer rows are
shut ofi; When a small head only is available the outer row, or
ring of vanes, is used.

The mixed flow turbine may also be used with a suction tube,
will govern accurately, and has a still higher efficiency than a
Jonval turbine. An efficiency as high as 86 per cent, is often
obtained with a turbine of this type. The mixed flow turbine
is much used in America ; it may be of smaller dimensions for a
given power than turbines of other types. The " Hercules " and
"Little Giant" turbines are of this type.

Impulse Wheels. — These are frequently employed in moun-
tainous countries, and are very suitable for use in cases where
there is a very high fall of water, and where a moderate amount
of power only is required. The most common form of impulse
turbine is the Pelton wheel; this consists merely of a wheel,
around the circumference of which are a number of buckets. In
order to avoid shock when the water strikes the buckets, these
are constructed in the form of the letter W, the upper part of
the W points towards the jet, the jet strikes the central portion
and glides down to the bottom of the bucket ; the energy of the
water is thus given up without shock. A Pelton wheel can be
governed with great ease by throttling the jet. The largest
wheel of this type which has been constructed gives about 700
B.H.P. The efficiency of a well-constructed Pelton wheel is from
80 to 85 per cent.

Before proceeding further, it may be well to show the student
how to calculate the power which can be obtained with any

226 MBCHANIOAL BHGIKKBRINO FOR BEGINNERS.

given quantity of water falling through a given distance in a
certain time. We have seen that 1 H.P. is equivalent to raising
33,000 lbs. 1 foot high in one minute. Now, if a body of water,
weighing 33,000 lbs., falls through a distance of 1 foot, it is also
the equivalent of 1 H.F., and if there were no loss in the
turbine, 1 H.P. would be obtained from this quantity of water
falling through the distance mentioned. Admitting, however,
that there is a loss in the turbine, and assuming that the
efficiency of the turbine is 75 per cent., then, if we multiply
1 H.P. by 75, and divide by 100, we get the actual horee-power
—viz., -75, or I of 1 H.P.

The hydraulic horse-power given by a certain quantity of
water falling through a certain distance may, therefore, be
obtained by the following formula: —

C X 62-4 X F

33,000 -^-^-^

where C = cubic feet of water per minute.
F = fall in feet.
H.P. = hydraulic horse-power.

If this hydraulic horse-power is multiplied by the efficiency of
the turbine or water wheel, the actual or brake horse-power
available for external work will be obtained.

For instance, if the hydraulic horse-power is 120 H.P., and
the efficiency of the turoine is 80 per cent., the actual horse-
power will be 120 X 80 -^ 100, or 96 B.H.P. The reason for
multiplying C by 62*4 is because a cubic foot of water weighs
approximately 62*4 lbs. If the amount of water available is
given in cubic feet of water per second, the figure must be
multiplied by 60 to convert it into cubic feet per minute, or
else the 33,000 lbs. must be divided by 60.

Example, — The amount of water delivered to each of the Niagara Falls
turhines is 430 cuhic feet per second, or 25,800 cuhic feet per minute ; the
mean fall is 136 feet. What horse-power should be obtained ? The calcu-
lation is

26,800 X 62-4 x 136 « oo>i u j i- u

— ^^ = 6,634 hydraulic horse-power.

ou,OUU

If we assume the efficiencv of the turbines to be 75^ per cent., the
actual horse-power available is 5,010. The turbines are supposed to
give 6,000 B.H.P.

Suotion Tube. — We have said that the use of a suction tube
enables a turbine to be placed, within limits, near the upper
level of the water. A suction tube, however, has no useful
effect if it is over 30 feet in length, even if it is of small size ;

HYDRAULIC MACHINEBy. 227

with a suction tube of 5 feet diameter the limit of useful length
is about 19 or 20 feet, with a tube 10 feet in diameter the tube
should not be more than 10 or 12 feet long.

Governing. — The governing of a turbine with sufficient
accuracy to make it suitable for electric lighting purposes is not
altogether an easy task. If the regulation is effected by means
of a sluice valve placed at the outlet of the suction tube, such a
sluice valve requires to be opened or closed to a considerable
extent before it mat-erially affects the quantity of water passing.
In some small turbines of the Jonval type the governing is
effected by a device which rolls and unrolls a scroll of leather
belting over the inlet vanes, thus cutting a certain number out
of use when the turbine runs too fast. In the case of the
Niagara Falls turbines the governing is effected by means of a
circular sluice running right round the vanes, as shown by the
letter S in Fig. 97. The sluice is shown fully open ; by raising
it the whole outflow of water can be stopped. In the Niagara
turbines the vanes are divided into three equal portions which are
successively closed as the sluice is raised. The vertical shaft of
each of these turbines is provided with a flywheel 14 feet 6 inches
in diameter, weighing 10 tons ; the speed of the turbines is 250
revs, per minute. The rim of the flywheel is made of wrought
iron, to enable it to withstand the stress due to centrifugal force.
The reader may find it interesting to work out what the stress
in the rim amounts to, by the rule given in Chapter vii.

As considerable power is required to move the sluices, the
power is not taken from a centrifugal governor but from the
turbine itself; a centrifugal governor is used to throw the
regulating mechanism in and out of gear. At Niagara this
arrangement was not found to act quickly enough to ensure very
good governing, and at No. 2 power-house the sluices of the
turbines are worked by separate hydraulic ram, the ram itself
being controlled by a centrifugal governor. The sluices worked
in this way can be completely opened or closed in ten seconds ;
the ordinary variation of speed does not exceed 1 per cent., and
the momentary variation when the whole load is thrown off does
not exceed 5 per cent.

In some turbines the vanes themselves are opened and closed
in order to regulate the speed, but this method of governing
introduces a certain amount of complication whidhi is not
altogether desirable in a turbine working under water.

Bearings. — The bearings of turbines of small size are often
made of lignum vitae and require no lubrication beyond that of the
water ; the friction, however, is greater than with metal bearings.
With the latter it is usual to force in oil by means of a force

228 MECHANICAL ENGINEEBING FOR BEGINNERS.

pump. In many cases an over-head bearing is used which is not
in contact with the water.

Niagara Falls Turbines. — These turbines, which were the
largest in the world when constructed, have many interesting
features. The turbines, ten in number, are designed to give
5,000 H.P. each, they are placed in a tunnel at the bottom of the
fall, about 140 feet below the level of the room in which the electric
generators are fixed. The turbines in No. 1 power-house have a
double outflow, and are illustrated more or less diagrammatically
by Fig. 97. They are constructed so that the pressure of the
water in the turbine carries the great weight of the vertical
shaft) flywheel, and rotating portion of the electric generator,,
amounting altogether to about 70 tons. The lower internal
portion of the turbine casing is fixed and carries the downward
pressure due to the column of water ; the upper portion of the-
turbine casing has apertures so that the pressure of the water
supports the upper moving wheel which carries the shaft,
flywheel, and rotating portion of the dynamo. The mean fall is
136 feet, or, say, 130 feet, to the top of the turbine, and as every
foot of a vertical column of water gives a pressure of '434 lb. per
square inch, the pressure under the moving wheel of the turbine is
about 56 lbs. per square inch. The diameter of the moving upper
portion of the turbine is 6 feet 3 inches, the area being 4,417
inches; multiplying this by 56 lbs. we find that the upward
pressure of the water is over 100 tons. The shafts of these
turbines consist of hollow steel tubes 38 inches in diameter and
f inch thick ; these are lighter than solid shafts. The hollow
shafts give place, however, to solid shafts 10 and 11 inches in
diameter in the bearings.

The latter turbines installed in No. 2 power-house at Niagara
are of the inward flow type.

Falls of Foyers Installation. — The largest turbine instal-
lation in this country is at the Falls of Foyers in Scotland.
There are five impulse turbines with vertical shafts, each turbine
gives 700 B.H.P. under a fall of 350 feet. The turbines are
9 feet in diameter and their speed is 140 revs, per minute.

Pumps for raising water are usually of the centrifugal or
reciprocating types, but there is a third form of pump of which
the best known example is the Pulsometer.

Centrifugal Pumps. — A centrifugal pump is shown by
Fig. 98. This form of pump is suitable for dealing with large
quantities of water when the height to which it has to be raised, or
the head, is not very great. For heads up to 5 or 6 feet, a centri-
fugal pump is considered to be more efficient than any other type
of pump ; for heads up to 20 feet, it is considered to be as efficient

HYDRAULIC MACHINERY.

229

as a good reciprocating pump ; but when the head is over 20 feet
a centrifugal pump is not so efficient as a good reciprocating
pump, but it bias other advantages, such as small first cost, small
space occupied, ability to deal with muddy and gritty water, <fec.
A centrifugal pump with a single impeller will raise water up
to 60 or 70 feet, and by placing two such pumps in series — i.e.,
one pumping into the other — water can be raised to a height of
140 feet or more. - Pumps arranged in this way would not, how*
ever, work at the highest efficiency, and in cases where the head
is greater than 70 feet the pump is usually constructed with two
or more impellers in one casing, so that the passage from the

Fig. 98. — Centrifugal pump.

periphery of one impeller to the inlet of the next is as short and
direct as possible.

High-speed multiple-stage centrifugal pumps, driven either by
steam turbines or by electric motors, for delivering water against
heads up to 500 and 600 feet are quite common on the Continent,
and it is said that by coupling in series two four-stage pumps
Messrs. Sultzer have been able to deliver water against a head of
1,700 feet. The pumps in question were driven by three-phase
motors, the speed of motors and pumps being 1,040 revs, per
minute.

A centrifugal pump, after having been charged, will continue

230

MECHANICAL ENOINEERINO FOB BEGINNERS.

to draw its water from a depth of 25 or 26 feet below the centre
of the pump, provided the joints of the suction pipe are all tight,
but such a great suction lift is not desirable. The suction lift
should not be more than 12 or 15 feet if practicable.

The efficiency of a well-designed centrifugal pump working
under suitable conditions is from 70 to 76 per cent.

The following table gives the approximate number of gallons
delivered per minute by some centrifugal pumps of Messrs.
Gwynnes' make, also the speeds at which the ]»umps must be
driven to deliver water against various heads : — .

TABLE XTX-

Size of suction 1

and discharge -

10"

18"

20"

pipes, . .J

A n T\ t*rk v 1 m a ^ £kS

.ZxppiUAlIIltli I/O

number of

gallons die- -

184

325

730

1,300

2,000

2,930

7,000

8,600

charged per

minute. J

Revolutions per

minute for —

10-foot head,

900

676

522

450

268

20- „ ,.

1,160

1.160

876

677

584

350

360

40- ,, ,,

1,646

1,546

1,161

897

773

463

50- „ „

1,700

1,275

986

849

500

600

70. „ „

1,963

1,473

1,139

981

687

587

The speeds given in the table are those at which the standard
pumps will deliver the quantities of water mentioned at the best
efficiencies, and will give the beginner an idea as to the speed at
which such pumps must be driven, but centrifugal pumps can be
specially designed for lower or higher speeds. Thus, if it were
desired to couple a pump direct to a vertical engine, the speeds
given in the table would be too high, and a pump having a larger
disc to run at a lower speed would be supplied ; if coupled to a
steam turbine, the speed would be too low, and a pump having a
smaller disc would be designed. There are, however, certain
limits in each direction ; if worked outside these, the efficiency
of the pump falls off very rapidly.

Fig. 98 is almost self-explanatory. All that need be said is
that the pump must be charged with water before starting ; a
foot valve is provided to prevent the charging water running
away, and to avoid the necessity of recharging the pump every
timie it is stopped and started. The water enters and leaves

HTDBAULIO MACHINERY. 231

the pump as shown by the arrows ; the rotation of the impeller
or disc with curved vanes imparts the necessary motion to the
water.

The illustration shows a pump with shrouded disc — i,e,, one in
which there is a rim of metal on each side of the vanes. This
shrouding has the effect of reducing the friction between the
water which is being rotated and the sides of the pump ; it also
strengthens the vanes. In un shrouded or open type disc pumps,
the vanes are strengthened by central ribs or fins. In some
shrouded pumps every alternate vane is not carried right down
to the boss, but ends at the shrouding ; this gives a freer entry
to the disc.

Beoiprocating Pumps. — All reciprocating pumps work on
practically the same principle. The bucket or plunger draws
water into the pump barrel through suction valves during one
stroke, and forces it out through discharge valves during the
next stroke. The chief differences between reciprocating pumps
are the manner in which they are driven ; the form of water
piston — i.e., whether bucket or plunger; and the design and
arrangement of the valves. A horizontal reciprocating pump of
the direct-acting type has been illustrated by Fig. 21, and the
somewhat uneconomical method of driving it referred to. A
vertical reciprocating pump, in which the valves are placed in
the bucket, is illustrated by Fig. 64. Large pumps are usually
driven by compound or triple-expansion engines, and extremely
economical results are obtained. A consumption of coal of
1 '4 lbs. per water horse-power is obtained in the best pumping
plants.

The water piston or bucket is frequently replaced by a plunger
or ram, which will displace the desired quantity of water per
stroke without the necessity of touching the sides of the pump
barrel. The plunger works through packing, and does not get
cut or scored in the same way as a bucket. Mine pumps and
other pumps which deal with gritty water almost invariably have
plungers, but in mines reciprocating pumps are being largely
superseded by centrifugal pumps.

Pumps must be placed within 25 feet of the level of the water
to be pumped, and if the water is hot the pump should be placed
below the level of the water ; but even when this is done, there
is usually some difficulty in pumping very hot water. The reason
is this— We have seen that the point at which water turns into
steam depends upon the pressure upon it, as well as upon the
heat it contains. Now, if a pump attempts to lift water at 200° F.
(and containing 1,142 B.T.U.), immediately the ram or bucket
moves quickly away from the water, the latter tends to lag

232 MECHANICAL ENGINEERING FOR BEGINNERS.

behind owing to inertia and to the friction in the pipes and
valves, and thus a partial vacuum is created ; the water then,
instead of following the ram, turns into steam, and the pump has
to deal with steam instead of water. It is for this reason that
feed pumps are always arranged to force the feed water through
an economiser, or an exhaust feed-water heater ; and not to draw
the water through, and then force it into the boiler.

The successful working of a pump depends very largely upon
the valves employed. The earliest form of valve, called a clack
valve, consisted of a leather flap ; the leather formed the hinge,
and the part of the leather which covered the opening was
strengthened by a metal plate. Such valves were only suitable
f6r low lifts and moderate speeds. A clack valve is sometimes
made with a metal hinge. The pin is a very loose fit in its seat,
so that the face of the valve may press tightly against the valve
seat. Such valves are sometimes used in mine pumps where the
water is very gritty.

A plain gun-metal disc valve, which rises off its seat and is
biought back by the pressure of water above it, and by a spring,
is used in hydraulic pressure pumps, but with this form of valve
the pump must run at a low speed, and the valve must have a
small lift ; otherwise, a heavy blow is struck every time the valve
closes. The duplex pump (Fig. 21) has, for the sake of illustrsr
tion, been shown with two different types of metal valves, while
three other types of valve have been shown on the drawing of
the air pump (Fig. 64).

.The head valves in Fig. 64 consist of a grid and guard, the
valve itself being of rubber. The guard is saucer-shaped, and
prevents the rubber valve from opening to too great an extent.
The centre of the rubber valve does not rise off its seat. This
type of valve is often used in air pumps ; it is unsuitable for
cases where there is a considerable head of water above the
valve.

- The bucket valves in the illustration consist of a disc of
vulcanised fibre protected on its back by a bronze disc. The
fibre valve rises off its seat, and is closed by the pressure of
water upon it, and by a spring. This is a good form of valve,
as the fibre does not require to bend, and it does not strike such
a heavy blow on its seat as is the case with a solid metallic
valve.

The foot valves shown are of the Klinghorn pattern; this
valve consists of three or more thin bronze discs; the discs, with
the exception of the upper one, are perforated, but in such a
manner that the perforations do not come opposite to one
another, so that when the discs are close together no water

HYDRAULIC MACHINERY.

233

passes, but when opened a slight distance the water can pass
through the perforations. The lift of each disc is very small,
and as they are thin and light, and as there is a small quantity
of water between each disc when the plunger reverses its stroke.

Fig. 99. — Pulsometer pump.

the valve closes very gently. Kinghorn valves are largely used
in marine air pumps, and are found to last a long time.

The Pulsometer Pump. — ^This pump which is shown in
section by Fig. 99 is of great simplicity, and is extremely useful
in cases of emergency. The pump can be lowered down a pit by

234 MECHANICAL ENGIKEBRINO FOR BEGINNERS.

a chain, steam being supplied to it by a flexible or ordinary
steam pipe. No foundations or fixings are required. The
pump consists of two chambers A, A, which join at the neck J.
In the neck is placed a single ball valve, I. The ball oscillates
on its seat, so that while the entrance to one chamber is open,
the entrance to the other is closed. The pump is first charged
with water, the steam enters one of the chambers which happens
to be left uncovered by the ball, it forces down the water until
it reaches the outlet shown by dotted lines, when the surface of
the water (previously covered by a film of air) is broken up, the
steam then blows through the outlet with a certain amount of
violence, comes into intimate contact with a large amount of
cold water and condenses, a vacuum is formed, and the ball is
pulled over. The vacuum then causes water to rise up through
the suction valves E, E, into the chamber just emptied; it also
draws in a small supply of air through a snifting valve not
shown by the illustration, but placed high up in the narrow part
of the chamber. The air thus draivn in /orms a cushion betioeen
the steam and the water on the succeeding stroke. While this has
been going on, the operation first described has been proceeding
in the second chamber.

The snifting valves which admit a small supply of air at
every stroke are seldom if ever referred to in published descrip-
tions of the pump, or by lecturers, yet they are a very important
feature. The pump, it is true, will work without them, but the
difference in the consumption of steam when these valves are
removed, and their holes plugged with wood, is at once notice-
able.

The chamber B in the illustration is merely an air-vessel
connected to the suction, and does not affect the principle of
the pump. The air-vessel is similar to those often fitted to the
discharge of reciprocating pumps for ensuring a fairly continuous
flow of water; at every stroke the small quantity of air im-
prisoned in the vessel is compressed; between strokes the air
expands and forces out some of the water. In the pulsometer
the air-vessel ensures a regular supply of water.

The pulsometer having no working parts is not affected by
grit; it will deal with semi-liquids such as mud, liquid cement,
sewage-sludge, &c. It requires no oil; the steam used is
condensed, and is therefore not a nuisance when the pump is
worked down a pit or in a confined space. The pulsometer will
raise water to a height of from 70 to 80 feet; it should be placed
within from 6 to 15 feet of the surface of the water, depending
upon the size of the pump. The only objection to this form of
pump is that its consumption of steam is higher than that of a

HYDRAULIC MACHINERY. 235

good reciprocating pump. The patents in connection with the
pulsometer having expired, a good many pumps on similar lines
are now made, such as the Aqua-thruster, the Expulsor pump,
and others. Whether these pumps are as good as the original
pulsometer or not the author is unable to say; he has not had
one under his own observation.

An ingenious device for enabling a large quantity of water at
a low level to raise a small quantity to a higher level is called a
Hydraulic Ram. The action is this : — Water flows down a long
pipe and is allowed to run to waste through a valve which is
kept open by a weak spring; when the water has attained a
sufficient velocity it closes the valve, and forces open a second
valve, which admits to the pipe leading to the higher level ; the
kinetic energy acquired by the water is sufficiently great to
carry a small quantity to the higher level. When the velocity
of tne water falls off, the waste valve again opens, and the cycle
is repeated.

The materials of which pumps should be made for pumping
special liquids require to be carefully considered. The following
have worked well: —

Amoniacal liquor.

Naphtha, .

Tar and creosote,

Petroleum,

Weak acids,

Sugar, treacle, and malt,

Vinegar,

Salt water.

Cast iron entirely.

>» >>

>> >j

Cast iron and brass.
Gun-metal.

i>
Lead.
Copper 88, tin 10, zinc 2.

Plow of Water in Long Pipes. — A formula which gives
the head necessary to overcomfe the friction of water in pipes is
sure to be very useful to students. This formula enables an
engineer to decide whether a pipe of a given size is, or is not,
large enough to pass a certain quantity of water in a given time.

The formula, which is given in Box's useful book on hydraulics,
is as follows: —

(3D)6 '

where G = gallons per minute.

L = length of pipe in yards.
D = diameter of pipe m inches.
H = head in feet.

As a table of the fifth powers of numbers is not always

236

MECHANICAL EKGINEERING FOB BEOINNERS.

available, and the formula is such a useful one, a table of fifth
powers is appended : —

TABLE XX.— Fifth Power of Numbers.

No.

5th Power.

No.

5th Power.

No.

5th Power.

45.435,424

1,350,125.107

62,621,875

1,453.933,668

243

60,466,176

1,664,031,349

1,024

69,343,957

1.680,700.000

3,125

79,235,168

1,804.229,351

7.776

90,224,199

1.934.917.632

16,807

102.400,000

2,073,071,693

32,768

115,856,201

2,219,006,624

59,094

130,691,232

2,373,046,875

100,000

147,008,443

2,535,525,376

161,051

164,916,222

2,706.784,167

248,832

184,628,125

2,887,174,368

371.293

205,962,976

3,077,056,399

537,824

229.345,007

3,276,800,000

759,375

264,803,968

3.486,784,401

1,048,576

282,476.249

3,707.398,432

1,419,857

312,500.000

3.939,040,643

1,889,568

345,025.261

4,182,119,424

2,476,099

380,204,032

4,437,053.125

3,200,000

418,195,493

4,704,270,176

4,084,101

459,165.024

4,984,209,207

6,153,632

503,284,376

5,277.319,168

6,436,343

650,731,776

5,584,059,449

7.962,624

601,692,067

6,904,900,000

9,765,624

666,366,768

6,240.321.451

11,881,376

714,924,299

6,690,815,232

14.348,907

777,600,000

6,956,883,693

17,210,368

844,696,301

7,339,040,224

20,511,149

916,132,832

7,737,809,375

24,300.000

992,436,543

8.153,726,976

28,629,151

1,073,741,824

8,587.340,257

33,554.432

1,160.290,625

9.039,207,968

39,135,393

1,252,332,676

9,509,900,499

Example. — Suppose we have a pipe 80 yards long and only 6 inches in
diameter, and we wish to pass 1.000 gallons of water per minute through
it. what head or pressure will be required to make this quantity of water
flow through the pipe ? The calculation will be as follows : —

1,000^ X 80

(3 X 6)5 *

The square of 1,000 is 1,000,000. and multiplying this by 80 we get—
80,000,000
(18)* •

HYDRAULIC MACHINERY. 237

The fifth power of 18 is 1,889,668, so we get—
80,000,000

1,889,568

= say 42 feet.

= 1-33 feet.

The answer shows that a head of 42 feet would be required to force
1,000 gallons of water through a pipe 6 inches in diameter and 80 yards
long. Such a head would be quite inadmissible under ordinary conditions,
and the calculation shows, either that the quantity of water would have
to be reduced or a larger pipe used.

Now let us try the effect of passing tlie same quantity of water per
minute through a pipe 12 inches in diameter.

1,000=^ X 80
(3 X 12)5 '

80,000,000

80,000,000
°^ 60,460,176

A head of 1 *33 feet, corresponding with a pressure of about '67 lb. per
square inch, would be a reasonable one to allow for forcing the water
through the pipe, assuming the water has to be pumped ; if, on the other
hand, a natural head of between 6 and 6 feet is available, the reader will
find that according to the above formula a pipe 9 inches in diameter is
suflBciently large.

USEFUL HYDRAULIC MEMORANDA.

cubic foot of fresh water at 32° F. weighs 62*418 lbs.

„ *39-r-40'' „ 62-425

60° „ 62-321

100° „ 62-022

200° „ 60 081

gallon of fresh water at 60° „ 10

„ = 277-27 cubic inches.

lb. of water at 32° measures 27-68 cubic inches.

60° „ 27-72

A column of water at 40° F. 1 foot high = -4335 lb. per sq. in.

60° „ =-4328

A X L
The capacity of a cylinder in gallons = ^^ ;

where A = area of cylinder in inches.
L = length „ „

Pressure of water at 40° F. on the side of a vessel
= A X D X 62-425 lbs.;

where A = area of side in feet.
D = half depth in feet.

* Water is at its greatest density at a temperature of 39* 1°.

239

CHAPTER XIII.
GAS AND OIL ENGINES.

SUCTION GAS PLANT.

In gas and oil engines the fuel is burnt in the cylinders, hence
they are called internal combustion engines, as distinguished
from external combustion engines, such as those driven by
steam, in which the fuel is burnt in a furnace outside the
engine.

Burning the fuel in the cylinder enables a higher thermal
efficiency to be obtained than is possible with an external com-
bustion engine, but introduces difficulties which are absent in
the latter. The advantages and disadvantages of the gas engine
will be discussed later. In the meantime the principles upon
which gas engines usually work will be described.

Otto Cycle. — In the great majority of gas and oil engines
the Otto cycle or four-stroke cycle is adopted ; with this cycle
one explosion is obtained during every two revolutions of the
engine crank. The action, commencing with the explosion, is as
follows : —

. 1. Explosion — Outward stroke.

2. Inward stroke — Piston drives out exhaust gases.

3. Outward stroke — Piston draws in mixture of gas and air.

4. Inward stroke — Piston compresses mixture.

When the cycle is again repeated.

Description of Engine. — Fig. 100 represents a 10 B.H.P.
gas engine * stripped of all details, such as governor, lubricators,
cams, levers, &c. The piston is shown at the beginning of its
stroke, having compressed its charge ready for ignition. After
the explosion has taken place, and the piston has reached the
end of its stroke, the exhaust valve will be lifted by a lever
worked from a cam on the shaft, a portion of which is shown
at the front end of the engine. This shaft is driven by gearing,
and only makes one revolution for every two of the engine.
After the burnt gases have been driven out, the gas and air
valves, which work horizontally, are opened by levers worked
from cams on the shaft already referred to, and a charge of gas

* Constructed by the Railway and General Engineering Company of
Nottingham.

240

MECHANICAL ENGINEEBING FOR BEGINNERS.

and air is drawn in; this mixture is then compressed by the
piston on its return stroke, when all is ready for ignition.

The engine illustrated has many good features. In the first
place, the valves and their seatings can easily be removed for

examination and cleaning ; in the second place, the cylinder and
water-jacket are not overhung and carried entirely by the flange
at one end, as is so commonly met with. It may be considered
by some that valves working horizontally, as the gas and air

GAS AND OIL ENGINES. 241

valves in the engine illustrated, ar^ not so good as those which
have a vertical lift ; in practice, however, the horizontal valves
have been found to work well.

The points about a gas engine to which attention may be
directed, taking them in the order of the cycle already given,
are as follows : —

Methods of Ignition. — In the earlier Otto engines, in which
a single slide valve was used for admitting the mixture and
exhausting the burnt gas, the slide valve brought forward at the
right moment a small pocketful of lighted gas, and placed it
opposite a port at the end of the cylinder, the result being that
the compressed mixture immediately exploded. The slide valve
with its rubbing surfaces was found to be unsuitable for high
speeds and for high temperatures and pressures, and was aban-
doned in favour of valves of the mushroom type, as shown by
the illustration. When the mushroom valves were adopted, the
tube method of ignition came into general use. This method
consists of keeping a tube red hot by allowing a flame to play
constantly upon its outside ; when the mixture has been
compressed, a small quantity is admitted to the inside of the
tube, and ignition takes place. This method of ignition is still
largely used, but in large gas engines, and in many modern
small gas and oil engines^ the electric method of ignition is
employed.

Electric ignition may be carried out in several ways. In
the majority of cases a small dynamo, called a magneto, is used.
If the current produced by the magneto is of low tension — the
type usually employed with gas engines — then the current is
suddenly interrupted in a place which is accessible to the
explosive mixture ; this sudden breaking of the circuit causes
a spark which fires the mixture. If the magneto is of the high-
tension type, as frequently used with petrol engines, the circuit
is made and broken outside the cylinder, the spark inside the
cylinder or explosion chamber being caused by the current
jumping across the fixed points of a sparking plug screwed
into the cylinder.

When a magneto is not used, a 2-cell accumulator giving a
current at an E.M.F. of about 4 volts is generally employed,
and an induction coil is used to increase the E.M.F. of the
current, so as to enable it to jump across the points of the
sparking plug. The current in such cases is interrupted by
means of a trembler placed at the top of the induction coil.
Such accumulators require to be charged periodically, and are
much more troublesome than a magneto.

In the Diesel oil engine no ignition apparatus of any kind is

242 MECHANICAL ENGINEERING FOR BEGINNERS.

used ; the air is compressed to a pressure of about 500 lbs. per
square inch, corresponding with a temperature of about 1,000'
F., which temperature is sufficiently high to fire the fuel
inmiediately it is injected.

Szhausting and Soayenging. — The mere fact of opening the
exhaust valve is not sufficient to ensure that all the burnt gases
will leave the cylinder; in fact, the clearances will remain filled
with them. These remaining products of combustion reduce
the value of the next incoming explosive mixture, and, if any
incandescent particles are left in the cylinder, they may cause
pre-ignition of the charge. To ensure the removal of the burnt
gases various expedients, called scavenging, are adopted. The
simplest is that usually employed in connection with the
Crossley engine ; it consists of an exhaust pipe of suitable
diameter 60 or 65 feet long. The exhaust gases travel along this
pipe at such a speed that (to express it colloquially) they find it
difficult to stop, and actually create a vacuum in the cylinder.

This arrangement is not altogether satisfactory at light loads,
and in large gas engines it is not practicable to arrange for a
suitable length and diameter of exhaust pipe. Air pumps are
therefore frequently used for forcing air into the cylinders and
thus scavenging them before the fresh charge is admitted. The
air pumps are driven by some reciprocating part of the engine or
by an auxiliary crank.

Drawing in Mixture of Gas and Air and Governing. — A
gas engine may be governed in three ways — (1) By omitting to
open the gas inlet valve when the speed of the engine rises
beyond a certain point ; this is called governing on the hit-and-
miss principle, the miss occurring when the speed is too high. (2)
By altering the mixture of gas and air. (3) By altering the
quantity of the mixture admitted without altering the quality.
The first method is that usually adopted in small and medium
powered gas engines ; it is simple, but the governing is not
very accurate unless a really heavy flywheel is used. Governing
by either the second or third methods is preferable — ^viz., by
altering the quality or quantity of the mixture. By reducing
the quantity of the mixture admitted to the cylinder, the
compression is reduced and the full value of the explosive
mixture is not obtained. On the other hand, by reducing the
amount of gas admitted and so altering the quality of the
mixture, the gas is not burnt to the best advantage. Both
systems have their advocates.

Compressing the Mixture. — It has been conclusively proved
that a high degree of compression results in a high explosive
force, and vice versd; a mixture which is highly compressed

GAS AND OIL ENGINES.

243

burns more quickly than one which is only slightly compressed.
Consequently, during recent years the clearances in gas engines
have been reduced, and the compression increased. The amount
to which the charge can safely be compressed without risk of
pre-ignition, depends upon the scavenging and water-cooling
arrangements.

In modern gas engines of large power, and even in small
engines in which economy is studied, the compression ranges
from 150 to 200 lbs. per square inch. In the Diesel engine, the
air is compressed to about 500 lbs. ; the fuel is then injected by
a blast of air at a pressure of about 600 to 650 lbs., when the
mixture immediately ignites.

We have said that the majority of gas engines work on the

■^Afr

Exhaust Ports.
Fig. 101. — Korting gas engine.

Otto cycle ; the most noteworthy exception is the Korting engine,
shown by Fig. 101. This engine is double-acting, and there are
no exhaust valves. The exhaust ports are placed in the middle
of the cylinder, as shown by dotted lines; when the piston
reaches the end of its stroke in either direction, it uncovers the
exhaust ports ; as soon as the ports are uncovered, a charge of
fresh air is blown in, and the burnt gases are expelled. When
the piston has moved sufficiently far to close the exhaust ports,
a supply of gas is pumped in, the air and gas inlet valve then
closes, and the mixture is compressed.

By this arrangement there is an impulse every outward stroke,
and as the engine is double-acting, there are two impulses per
revolution. Such an engine occupies much less space than a
single-cylinder engine, in which there is only one impulse during

244 MECHANICAL BKGIKBBBING FOB BBOIMNER8.

every two revolutions. The gas and air pumps are placed along-
side the cylinder, and are worked off an auxiliary crank ; they
are not shown in the illustration.

Double-acting engines, working on the four-stroke cycle, are
frequently made on the Continent. These engines occupy less
space than those of the single-acting type, but the parts are not
quite so accessible for cleaning ; moreover, a gland is required at
the piston-rod end of the cylinder. It will be noticed that in the
single-acting engine, illustrated by Fig. 100, one side of the piston
or trunk is open to the air, and no gland is required ; the piston
can be withdrawn by undoing the bolts of the connecting-rod
brasses. These are good features. The gland of a double-acting
gas engine, unless designed with great care, and water-cooled, is
likely to be a source of trouble.

In double-acting engines, and in single-acting engines when
the power exceeds 250 B.H.P., it is advisable to water-cool the
piston and rod ; water for this purpose is admitted to the cross-
head by means of swinging link pipes, and thence through the
hollow piston-rod to the piston.

Amoiint of Gkts Consumed. — The amount of gas consumed
in an engine per horse-power developed, depends largely upon
the richness or otherwise of the gas. The number of British
thermal units contained in gases made by different processes is
approximately as follows : —

Town gas, . . . 600 to 680 B.T.U. per cubic foot.
Dowson gas, . . 150 to 164 „ „

Producer gas (suction gas ]

plant), . . . > 125 to 155 „ „

Mond gas, . . .J
Blast-furnace gas, . . 100 to 120 „ „

Town gas requires to be mixed with about 10 times its volume
of air, while producer gas requires about IJ or 1| times its
volume of air, in order to make the best explosive mixture in
the engine. The consumption of rich town gas in an engine of,
say, 20 H.P. should be between 14 and 16 cubic feet per
I.H.P. per hour, which is equivalent to between 16J and
19 cubic feet per B.H.P. The consumption of producer gas
containing 130 or 140 B.T.U. in a good engine should not
exceed about 60 cubic feet per I.H.P. per hour, or 70 cubic
feet per B.H.P.

Horse-power of Qas Engines. — The indicated horse-power
of a gas engine, working on the Otto or any other cycle, may
be found, provided the mean pressure in the cylinder is known,
by the following formula : —

GAS AND OIL ENGINES. 2i5

S X N X A X P ,
33,000 '

where S = stroke of the piston in feet.

N = number of explosions per minute.

A = area of the piston in inches.

P = mean pressure exerted on the piston during the stroke.

Example, — What is the indicated horse-power of a gas engine having a
cylinder 10 inches in diameter, 18 inches stroke, and speed 190 revs, per
minute, the mean effective pressure on the piston being 70 lbs. ? The
calculation is

The above calculation is for an engine working on the Otto cycle, in
which there is an explosion for every 2 revs, of the engine.

The actual horse-power of a gas engine can, however, only be
determined with accuracy by a brake trial, or by coupling it to a
dynamo, the efficiency of which is known, for, in a gas engine,
the indicator cards which purport to give the mean pressure are
not altogether to be relied upon.

The maximum temperature attained in the interior of a gas-
engine cylinder by jising a mixture of 1 part of town gas to 9
parts of air, and compressing it to about 100 lbs. pressure, is
between 3,000° and 3,500" F. The higher of these temperatures
is above the melting point of platinum, so that the necessity of
a water-jacket is apparent. The maximum pressure reached in
the cylinder is about 3 or 3 J times the compression pressure.

Thermal Efficienoy. — The thermal efficiency, either per
indicated or per brake horse-power, of a gas engine is easily
ascertained if the number of cubic feet of gas used per horse-
power per hour, and the calorific value of the gas, are known.
Let us find the thermal efficiency per B.H.P. of a large gas
engine using 60 cubic feet of producer gas per I.H.P. per hour.
If the mechanical efficiency of the engine is between 85J and 86
per cent., the consumption of gas will be 70 cubic feet per B.H.P.
per hour. We will assume that the calorific value of the gas is
130 B.1\U. per cubic foot. The heat used by the engine per
B.H.P. is, therefore, 70 x 130 = 9,100 B.T.U. per hour. We
read in Chapter iii. that a B.T.U. = 772 ft.-lbs., and a H.P.
33,000 ft.-lbs; therefore, 42-75 B.T.U. per minute, or 2,565
B.T.U. per hour, are the equivalent of 1 H.P. The engine uses
9,100 B.T.U. per brake horse-power, so that the thermal effi-

2 565
ciency of the engine is * j. = -28. The absolute thermal

efficiency of the engine per B.H.P. is therefore -28.

246 MECHANICAL BN6INEEBING ^OB BEGINNERS.

If the engine only uses -28 of the heat supplied to it, the
question which naturally arises is — What hecomes of the
remainder of the heat ? The distribution of the heat originally
contained in the gas is approximately as follows : —

*38 carried off in the exhaust gases.

•29 carried off by the jacket cooling water by radiation.

•28 given up as brake horse-power.

•06 used in overcoming the friction of the engine.

1-00

Although a thermal efficiency of '28 is apparently low, it
compares favourably with that of a steam engine. The absolute
thermal efficiency of a triple-expansion condensing engine,
working with saturated steam at 160 lbs. pressure, and using
14 lbs. of steam per B.H.P., is only -162.

Speeds of Gas Engines. — The speeds at which gas engines
usually run are approximately as follows : —

5 B.H.P., 250 revs, per minute.

10 „ 220 „

20 „ 200 „

50 „ 175 „

100 „ 160 „

500 „ 120 „

Buction Ghas Plant. — ^The history of the modern suction
plant is briefly as follows : — It was found by the late Mr. Siemens
that it was more economical and convenient to turn cheap fuel
into gas, and then bum the gas in regenerative furnaces, than to
utilise the heat derivable from complete combustion of the coal
in the first place. To turn the coal into gas it was placed on a
deep layer on a grate and partially burnt, the air that found its
way through the coal not being of sufficient quantity to allow of
complete combustion. The gas that was given off consisted
chiefly of carbon monoxide (CO) and nitrogen, together with a
small quantity of hydrogen. The gas was of low calorific value,
and, in its production, not less than 30 per cent, of the heat
contained in the coal was lost. The small quantitjr of hydrogen,
too, which it contained prevented the rapid ignition which is
necessary in an explosive mixture. In order to improve the
quality of the gas, Mr. Dowson, in 1878, turned a jet of steam
(HgO) into the hot fuel, the result being that the hydrogen was
liberated and was added to the gas, while the oxygen combined
with the carbon. The temperature of the furnace was reduced

GAS AND Olh ENGINES. 247

and the quality of the gas improved, the calorific value of the
Dowson gas being within 15 per cent, of that of the solid coal.

Although gas produced on the Dowson system was of fair
calorific value and inexpensive, there were at least two reasons
why the system did not come into general use for driving gas
engines. The first was the high cost of the plant, and the
second the great amount of floor space required. A boiler,
working at about 50 lbs. pressure, was required to drive the
steam and air through the coal, and a gas-holder was provided
to contain the gas after it had been produced.

In a modern suction gas plant the heat generated in the
producer is used to form the steam, and, as the heat is not
sufficiently great to generate steam at a pressure sufficiently
high to force it through the fuel, the outward stroke of the
engine is used to suck the steam and air through the fire. A
gas-holder, if placed between the generator and the engine,
would do harm, as the eflfects of the suction stroke would not be
felt in the generator: hence both boiler and gas-holder have been
done away with.

One of the simplest forms of suction gas plants* now made is
shown by Fig. 102. The generator or producer A in which the
fuel is burnt is surrounded by a jacket or vaporiser, B; this
jacket is filled with coke or any similar material; the material is
kept moist by sprinkling water upon it, and is warmed by the
heat of the producer. The air required for combustion is com-
pelled to pass through the belt of wet coke in the jacket, and in
doing so becomes charged with moisture; the suction stroke of
the engine draws the hot moist air through the fuel, as already
jexplained. The amount of water sprinkled on to the coke in the
jacket is regulated automatically by the suction of the engine.
After the gas has been made in the producer, it is necessary to
pass it through the scrubber shown on the right-hand side of the
illustration. The scrubber consists merely of a cylinder filled
with coke, through which a stream of water is kept flowing in a
downward direction ; this water carries away any tar suspended
in the gas. After leaving the scrubber the gas is usually taken
to a box, which helps to reduce any violent fluctuations of
pressure, and is then taken to the engine. The fan shown on
the left side of the illustration is only used at starting, when the
cock C is opened, and the gases pass away to the atmosphere.

Amount of G-as made and Fewer developed per Found

of Coal. — A producer similar to the one illustrated will make

about 80 cubic feet of gas for every pound of anthracite coal

consumed, and as a good engine of fairly large size uses only

* Constructed by the Dowson Gas Co. of Westminster.

248

MECHANICAL BVOIKBERIirO FOR BEGINNERS.

about 70 cubic feet of producer gas per B.H.P., it follows that
1 lb. of anthracite will give about 1*14 B.H.P. It is the

Fig. 102. — Suction gas producer.

practice of many makers of suction gas plants to guarantee
that '9 lb. of anthracite will give 1 B.H.P., but at some

GAS AND OIL ENGINES.

249

trials of several 15 to 20 B.H.P. engines working with suction
gas, carried out by the Royal Agricultural Society in 1906, the
consumption was a little higher. The lowest consumption was
r04 lbs. of anthracite, and the highest 1-47 lbs., the average
consumption of the eleven engines being 1*21 lbs. of anthracite
per B.H.P. The consumption of an engine working with a
Dowson producer, as illustrated, was, at the trials in question,
1'09 lbs. per B.H.P. The average consumption of eleven plants
working with coke was 1*4 lbs. per B.H.P.

Water reqiiired for Suction Gas Plants. — It is usually
reckoned that about 1 gallon of water is required per B.H.P.,
nine-tenths of which is used in the scrubber, the remaining
tenth in the generator. In the trials carried out by the Royal
Agricultural Society, the consumption of water was considerably
higher than 1 gallon. The average consumption of twelve
plants was 1*89 gallons of water per B.H.P., or about three-
fourths as much as would be used by a good non-condensing
steam engine. The amount of water used by the plant which
secured the gold medal was 1'14 gallons per B.H.P.

Mend G-as.— Bituminous coal cannot be used in a suction
gas producer, only anthracite coal or coke. Dr. Mond, however,
has introduced a process by which cheap bituminous coal may
be used for the production of gas. The Mond process consists of
introducing an enormous quantity of superheated steam and air
to the generator. The greater portion of the steam passes out
of the producer undecomposed ; it is then condensed, and its
heat is utilised. The temperature of the generator is kept very
low by the introduction of the very large quantity of steam and
air, and a large amount of sulphate of ammonia is obtained ; this
forms a valuable by-product. No tar is produced. This system,
like that first introduced by Mr. Dowson, requires a separate
boiler. About 70 cubic feet of gas are produced from 1 lb. of
bituminous coal.

The analyses of gases produced by the Dowson, Suction, and Mond
processes are approximately as follows ; —

Dowson
System.

Suction Gas
System.

Mond
System.

Hydrogen (H), ....
Oxygen (0), . . . -.
Carbon monoxide (CO), .

„ dioxide (CO2),
Marsh gas (CH4), .
Nitrogen (N), ....

19-8

23-8
6-3
1-3

48-8

17-5 to 20-5
0-5 „ 0-2

18-5 „ 21-5
70 „ 7-5
1-5 „ -5

65-0 „ 49-8

24-8

13-2

12-9

2-3

46-8

1000

100-0 1000

100

250 MECHANICAL ENOIN EBBING FOR BEGINNERS.

Starting Gkts Engines. — Small gas engines may be started
by the driver pulling round the flywheel by hand, and thus
drawing in the explosive mixture and compressing it; but in
the case of large engines this is impracticable, and special means
for starting have to be provided. Some engines, such as the
Westinghouse gas engine and the Diesel oil engine, are started
by means of compressed air; the engine is provided with a pump
which compresses a certain quantity of air before stopping, and
forces it into a drum provided for the purpose. Other engines
are provided with a starter, consisting of a chamber into which
a mixture of gas and air is pumped by hand; the flywheel of the
engine is then barred round by a lever with a pawl at the end,
teeth being cast in the flywheel for the purpose. When the
piston is in the right position, the cylinder is placed in com-
munication with the starter, the mixture is flred, and the
resulting explosion in the cylinder is sufficient to start the
engine. Other engines merely have a pinion with large hand
wheel; the pinion works in mesh with teeth in the large fly-
wheel; the large hand- wheel is constructed on the free-wheel
principle, so that it is not driven round when the gas engine
is running.

Comparative Merits of Steam, Gkts, and Oil Engines. —
The cost of fuel per horse-power for gas, steam, oil, and petrol
engines is approximately as follows : —

Penny per B.H.P.
per hour.

Gas engine using producer gas, and consuming
1 J lbs. of coke per B.H.P. The cost of coke
is taken at 15s. per ton, ....•!

Gas engine using 20 cubic feet of town gas per
B.H.P. The cost of town gas is taken at
2s. 6d. per 1,000 feet, -6

Condensing steam engine using If lbs. of coal
per B.H.P. The cost of coal is taken at 15s.
per ton, '14

Non-condensing steam engine using 3-75 lbs. of
coal per B.H.P. The cost of coal is taken at
15s. per ton, "3

Oil engine using '085 of a gallon, at 4d. per

gallon, '34

Petrol engine using -11 of a gallon, at Is. 4d.

per gallon, , 1*76

The cost of coal per ton, of gas per 1,000 cubic feet, and of oil
and petrol per gallon are given so that the reader can make any

GAS AND OIL ENGINES. 251

necessary correction in the comparative costs to suit the cost of
fuel in his own locality.

If the water required for either steam or gas plant has to be
paid for, the cost must be added to the figures given.

It will be seen that, so far as fuel alone is concerned, a gas
engine using producer gas is the cheapest form of motive power,
also that a gas engine using town gas at 2s. 6d. per 1,000 feet is,
with the exception of a petrol engine, the most expensive of
those given. Fuel, however, is not the only item to be taken
into consideration ; there is, for instance, the cost of attendance.
A small gas engine running on town gas will run without
attention, whereas a steam boiler requires a man to stoke it and
to see that the water level is maintained. If we take the case
of a 10 horse-power gas engine running fifty-four hours per week
on town gas costing '6 of a penny per B.H.P. without attendance,
and a steam engine costing only '3 of a penny per B.H.P., but
necessitating 18s. per week being spent in wages, the comparison
will come out in favour of the gas engine. It is largely due to
the fact that small gas engines can be run practically without
attention, and can be stopped and started without any fuel being
used while the engine is standing, that they are so widely used
in small works. With large gas engines and producers the cost
of attend|knce is as great as, if not greater than, that required
with steam plant.

On grounds other than those of running costs and possible
risk of boiler explosions, the advantages are all on the side of
the steam engine. Ease of starting, silence, general sweetness
of running, and reliability, are qualities much more marked in
the steam engine than in its rival. The author a few years ago
spent some hours in an electric generating station where gas
engines were employed, and he subsequently revisited the same
station when the gas engines had been replaced by steam plant.
The strongest impressions produced at the first visit were the
terrible noise (it was quite impossible to hear oneself speak in
the engine-room), and the pallid appearance and anxious look
upon the faces of the attendants. The chief engineer said that
he was much handicapped by illness amongst his staff due to
noxious fumes.

On the second visit, when the generators were driven direct
by 600 H.P. single-acting engines, the whole conditions were
changed ; one could converse, without raising one's voice, in any
part of the engine-room, and the attendants all looked well;
and while during the first visit one's chief desire was to
escape as quickly as politeness permitted from what was
almost an inferno, on the second visit one was tempted to

252 MECHANICAL SNOINEBRINO FOR BEGINNERS.

linger unduly amongst the silent-running plant, and around the
switch board.

We have said that the steam engine is more reliable than the
gas engine. Perhaps this statement should be qualified in this
way. It is not suggested that a gas engine, provided its valves
are kept clean, and supplied with a proper explosive mixture, is
not reliable, but the author's meaning is this — given water, coal,
and a sound boiler one can be sure of getting steam of some
definite and easily ascertainable pressure, and if this steam fails
to drive the engine the fault can be located easily. But in the
case of a producer plant, given coal, water, and a producer,
one cannot be absolutely sure of getting a gas of some definite
explosive strength. On paper it is, of course, a very easy
matter, but in practice the strength of the explosive mixture
varies considerably, and unless the engine is of sufficient size to
give the full power required with a weak mixture, its speed will
fall off and the whole work of the factory may be deranged.
A small gas engine working with town gas does not experience
great variations in the quality of the gas supplied, and is pro-
bably as reliable as a steam engine.

Oil Engines work on the same principles as gas engines ; but
the former, with the exception of the Diesel engine, require some
form of vaporiser to turn the heavy oils used into vapour. The
difference between the various makes of oil . engines consists
chiefly in the vaporiser. In the Homsby engine, as shown by
the illustration, this forms a continuation of the cylinder, and
when compression is completed the temperature is sufficiently
high to fire the mixture without any ignition apparatus. A
lamp giving out great heat is used at starting, and is retained
until the vaporiser has become sufficiently warmed to vaporise
the oil.

In the Priestman engine the vaporiser is separate from the
cylinder, and oil is vaporised by being passed through a fine jet
and heated by the exhaust gases, while the mixture is fired by
electric ignition

The characteristics of the Diesel engine have been referred to
in the remarks on ignition and compression.

Petrol Engines. — Petrol engines work on precisely the same
principles as gas engines. The four-stroke cycle is generally
adopted in this country and abroad. In the United States, how-
ever, a certain number of cheap two-stroke cycl6 engines are
made for launches. Petrol, owing to its light character,
vaporises very easily ; the vaporisation is effected in a chamber
called a carburetter. The petrol is drawn up in the form of
spray through several fine openings by the suction of the engine

GAS AND OIL ENGINES.

253

254 MECHANICAL ENGINEERING ^OB BEGINNERS.

and mixes with a current of air also drawn up by the engine
suction.

Consumption of Oil and Petrol. — ^The amount of oil used
in a good oil engine is about '085 of a gallon or '7 of a lb. per
B.H.P. In the Diesel engine the consumption is as low as
•41 lb. in an engine of 100 H.P., but the first cost of this engine
is rather high. The cost of suitable Russian oil "naked on
wharf " is about 45s. a ton. The cost of barrels and carriage may
bring the cost up to £4 per ton, or, roughly, fourpence per gallon.
The consumption of petrol is about '1 to '125 of a gallon, or
•7 to -9 of a lb. per B.H.P. per hour.

Specific Gravity of Oils. — We have spoken of heavy and
light oils ; the specific gravity of crude petroleum is about 0*92
so that a gallon weighs about 9*2 lbs. The ordinary household
petroleum used for lamps weighs about 8*2 lbs. per gallon
(specific gravity 0*82) ; while petrol or petroleum spirit weighs
from 6*6 to 7*2 lbs. per gallon (specific gravity 0*66 to 0*72).

One lb. of crude petroleum contains about 20,000 B.T.U.,
while 1 lb. of petrol contains about 17,500 B.T.U.

Flash Point. — The point at which an oil will vaporise when
heated is termed its flash point — i.e., if an oil vaporises at 75'
F. its flash point is said to be 75°. An oil, the flash point of
which is below 73**, is not considered legally safe in this country,
and may not be stored without restrictions; efforts are being -
made to raise this flash point on account of the numerous accidents
which occur with cheap lighting oils. The flash point of heavy
petroleum, such as is used in oil engines, is about 95^

255

CHAPTER XIV.
STRENGTH OF BEAMS AND USEFUL INFORMATION.

Strength of Beams. — To describe elaborate methods for
calculating with extreme accuracy the strength of beams of
unusual section, either graphically or mathematically, would be
beyond the scope of this book, and, in point of fact, such
calculations are seldom made by the average draughtsman or
designer, unless engaged upon bridge or' crane work. It is,
however, necessary for every young engineer to be able to-
ascertain quickly and with fair accuracy, what stress is set
up in the flanges, or body of a beam or cantilever when
loaded in a given manner.

In those cases where the metal is distributed in the most
efficient manner to resist bending — i.e., in the form of an upper
and lower flange with a web joining the two flanges, as in
the case of a rolled steel joist, the stress in each flange can be
found very quickly and simply by the formulae given in
column 1 of Table xxi., and, if the area of the flange is known,
the stress per square inch is quickly ascertained. When the beam
is supported at both ends and loaded, the upper flange is in
compression, the lower flange in tension; and vice versd when
the beam is supported at one end only. The web is assumed
to give stiffness only when this method of calculation is adopted.

In cases where the beam is of solid rectangular or of round
section, the stress cannot be found so simply, as the relative
value of the metal must be considered; for it is evident that
metal situated midway between the top and bottom of the beam,
or at its neutral axis, is not so efficient as metal placed at the
greatest distance from the neutral axis. The breaking strength
of rectangular beams can, however, be found in the manner
indicated in the second column, and of beams of other sections
by substituting for B x D^ the values for these sections given
by the figures accompanying the illustrations.

It will be noticed in the case of solid rectangular beams,,
column 2, that the square of the depth is taken; the reason for
this being that the strength of a rectangular beam varies as the
square of its depth. It is obvious, however, that if the depth
is squared and multiplied by the breadth, the number of inches,
so found, multiplied by the safe or breaking stress of the metal,.

256 HECHAinGAL ENGINEBBINa FOB BBOINNEBS.

TABLE XXI.

(

^=i'

prirnno"

Stress in Bach Vlange

in Beams of I Section

(depth measured from

centre to centre of

flangeX

WL
"D

WL
2D

WL
4D

WL
8D

WL
12D

4-7 X R»

..,.(5^)

BD»-26cy

Breaking Weight, in

Cwts., of Plain £ec-

tangular Beams.

BD^K

2BD2K

4BD2K

8BD2K

12BD2K

BD2L

LiLa

Greatest

Shearing

Force.

W
W

W
2"

JW
2

W = weight applied.
L = length of beam in ins.
B = breadth of beam in ins.
D = depth of beam in ins.
K = coefficient.

= 65 to 90 cwts. for cast-
iron test pieces free
from blowholes.
= 45 to 50 cwts. for
ordinary cast - iron
structures.
= 90 to 95 cwts. for rolled
steel joists.

STRENGTH OF BEAMS AND USEFUI^ INFORMATION. 257

must not be taken as giving the load the beam will bear,
coefficients for the ultimate strength of various materials are
therefore used when this method of calculation is employed, and
are given at the foot of the table.

The coefficient usually given for the strength of cast iron,
although fairly applicable to ordinary cast structures, is too low,
provided the metal is sound, iree from blowholes, and free from
internal stresses set up in cooling. For instance, we know that
it is customary to specify that a bar of cast iron, 2 inches deep
1 inch wide, placed on supports 36 inches apart, shall carry a load
of 30 cwts. suspended from its centre, and occasionally it is
specified that such a bar of cast iron intended for cylinder
liners shall carry a load of 40 cwts., and these results are
obtained in actual practice. Now, if we work by the formula

J , which applies to the case — see illustration No. 3

— and take a coefficient of 46 cwts., as given in a well-known
pocket-book, the bar would break under a load of about 20| cwts.
only. Working by the rule given in another widely-read and
useful pocket-book the bar would break under a load of 24 cwts.

Some engineers, instead of taking BD^ and using a small
coefficient, prefer to take the' useful modulus of the section,
which is considered to be ^ BD^, and to use a coefficient six
times greater than when using BD^. In the case of the test
bar referred to, if the modulus of the section — viz., J BD^ — is
taken, the coefficient of rupture, assuming the bar breaks with
30 cwts., is just over 20 tons. If the bar breaks with 40 cwts.,
the coefficient of rupture is about 27 tons. These coefficients are
much higher than the ultimate tensile stress of cast iron, and
the explanation is to be sought in the plasticity of the metal.
Sir Benjamin Baker found that if a beam loaded in the centre
was turned over and over, it would break after a very few turns
under a load, which, under ordinary circumstances, the beam
would carry indefinitely.

The arrangement of balls in the illustrations shows how the
weight is placed — t.e., whether evenly distributed or placed in
the centre of the beam. In Nos. 5 and 6 the ends of the beam
are built into the walls or otherwise fixed; this 'renders the
beam stronger than if merely supported, as in the case of
Nos. 4 and 5.

The following examples will doubtless help to make the
formulas clear: —

Example 1. — The oast-steel arm of the large hydraulio riveter shown
by Fig. 90 is 12 feet long (12 feet gap) and is 46 inches deep from centre to

258

liECHAlflCAIi EKGINEEBINO FOB BBOlKlTBRB.

centre of flanges. A preMure of 100 tons is applied at the end ; what

stress will there be in the flanges? The formula -^ given in column 1,

100 X 144
opposite the first illustration, applies, and the calculation is — — « 320.

The stress in each flange is, therefore, 3*20 tons. The flange is 24 inches
wide and 6 inches thick, so that there are 144 square inches to withstand
the stress; the stress upon the metal will, therefore, be 2*22 tons per
square inch.

Example 2. — A rolled steel joist of I section 6 inches wide and 10 inches
deep (sav 9^ inches from centre to centre of flange) is built securely
into walls 10 feet apart, and is uniformly loaded with a total weight of
26*2 tons ; what stress will there be in each flange ? The formula given
in column 1 , opposite the sixth illustration, applies, and the calculation is

"io — oTF" — ^'^'^ ^^"^ ^^ ®*^^ flange. We see, from a sectional drawing

of the joist supplied by the makers, that there are about 4 square inches
in each flange, so that the stress in the flange is 6*9 tons per square inch.

Example S. — ^What weight suspended from the centre of a cast-iron bar
2 inches deep 1 inch wide, placed upon supports 36 inches apart, would
break it ? The formula given in column 2, opposite the third illustration,
applies, and, taking the coefficient value of I^ to be 70 cwts. , the calcu-
lation will be ^r^ =31*1 cwts.

Example 4- — What weight suspended from the centre of a cast-iron

round bar 3 inches diameter, placed upon supports 36 inches apart, would

break it ? The value of the metal in a round section is given by the eighth

illustration and accompanving figures — viz., 4*7 x R^. The calculation,

., , . 4 x 4-7 x l-5» x70 .^ .

therefore, is 55 = 123 cwts.

The following table, issued by Messrs. Measures Bros., gives
the safe permanent loads which their standard rolled-steel joists
are capable of bearing when the ends are built into walls 10 feet
apart, and when the load is evenly distributed : —

Size.

Weight per Foot.

Safe Load, in Tons,
for a 10-feet Span.

Inches.

Lbs.

6x5

9*8

8x6

130

8x6

150

9i X 4i

10 x44

27i

170

10 X 5

186

10 X 6

26-2

12 X 5

27*5

12 X 6

33*2

14 X 6

44*1

16 X 6

STRENGTH OP BEAMS AND USEFUL INFORMATION. 259

As the strength of a beam varies inversely as its length, it is
easy to calculate what a beam of a length other than 10 feet will
carry; thus, a beam 20 feet between walls will carry half the
weight given above. If the ends of the beam are merely sup-
ported, and not built into the walls, the beam will only carry
two-thirds of the weight given above (or in the ratio of 8 to 12).
The ratio of strengths of beams, variously loaded and supported,
will be seen by glancing at the illustrations and the formulae in
columns 1 and 2.

In connection with structural ironwork, the student may
occasionally find it useful to be able to ascertain the stress in
any part of a structure by the graphic method. Fig. 104 shows
this method applied to a jib crane having a weight suspended
from a hook at the end. Assuming the weight suspended from
A is 3 tons, and it is desired to know what stress will be set up
in the jib and tie bars, the method of procedure is as follows : —
Draw A B equal to the weight ; thus, if the weight is 3 tons, the
line on a large-scale drawing may be drawn 3 inches long. Draw
the horizontal line B C, then C A will show what stress there
is in the jib — i.e,, if C A is 4 inches long, the stress in the jib is
4 tons. To find the stress in the tie bars, draw the vertical line
C D, then D A will show the stress in the tie bars. If the bars
had been horizontal, as indicated by dotted lines, the stress in
them would have been shown by E A. The stress in the jib and
tie bars may also be found by simple arithmetic ; thus, the stress
in the jib is found by multiplying the weight carried by the
length of the jib, and dividing by the distance F G (not F X).
The stress in the bars is found by multiplying the weight carried
by the length of the bars, and dividing by the distance F G.
The student would do well to draw out a triangle to a fairly
large scale, take an imaginary weight, and check the graphical
method by arithmetical calculations.

The extremely simple methods given above apply only if the
weight is suspended from the end of the jib. If the chain
carrying the weight passes along the tie bars and is pulled at by
a winch, then it is necessary to take the resultant force which
arises from the pull on the chain at one end and the weight at
the other. This resultant force is found very easily by the
graphical method, as shown by Fig. 105. The chain is shown by
the dotted line ; mark off A B equal to the weight, and mark off
A C equal to the pull upon the chain ; as the stress in all parts
of the chain is the same, it is obvious that AC = AB. Complete
the parallelogram C A B D, and the resultant force is shown by
the line D A. If now we wish to find the stresses set up in the
tie bars and jib of a crane in which the chain passes along the

260

MECHANICAL ENQINEERING FOR BEOINKERS.

tie bars the method of doing so is shown by Fig. 106. The
stress in the tie bars is shown by the line M, and in the jib by
the line P.

L^ £_ 2€

2 tons-

Fig 105. Fig. 106.

Figs. 104 to 106. — Graphic methods of computing stresses,

A point to be borne in mind in the construction of cranes is
this — If the angle formed by the more or less horizontal portion
of the chain and the jib is greater than the angle formed by the

STRENGTH OP BEAMS AND USEFUL INFORMATION. 261

jib and vertical portion of the chain, then the tie bars will be in
compression ; if less, then the tie bars will be in tension.

Amongst the wharfs and warehouses of big cities one occasion-
ally sees cranes with the tie bars bent. This is because such
cranes were designed by men without the knowledge of the
simple fact just mentioned; and small round tie bars, which
would be quite suitable for a tensile stress, have been subjected
to compression and have consequently bent. The author met
one crane-maker who refused to believe that the tie bars of a jib
crane could ever be in compression, and it was not until ocular
proof was given by means of a lawn-tennis post placed as a jib,
and a piece of cord and weight representing the chain, that the
crane-maker was convinced. He had attributed the fact of the
tie bars of his cranes becoming bent to the practice of men
crawling along ^em to oil the sheave at the end ! If the jib is
placed at an angle of 45** both in respect to the horizontal and
vertical portions of the chain, then the resultant thrust is entirely
taken by the jib, and the tie bars are neither in tension nor com-
pression ; but, of course, any swing of the chain will upset this
equilibrium at once.

The pull on the wall at the top bracket of a crane equals the
weight multiplied by the radius of the crane, and divided by the
height of the crane post.

Sundry Useful Information. — All questions relating to the
power transmitted by a screw, lever, wedge, or toggle joint, can
be answered by the following formula : —

M ^ m

where P = power transmitted.
p = power applied.
M = motion of power transmitted,
m = „ „ applied.

Example. — The hand- wheel of a stop valve is 30 inches in circumference,

and, for the sake of a simple example, we will assume that it is keyed to a

screwed spindle having only one thread per inch, so that during one

revolution of the hand- wheel the valve will be raised or lowered 1 inch.

A force of 10 lbs. is exerted at one portion of the hand- wheel, what power

will be transmitted to the valve ? The power applied will move through

30 inches, while the power transmitted will move through 1 inch. The

10 X 30
calculation, therefore, is = = 300. The power transmitted to the

valve is 300 lbs.

Questions arising in connection with the use of levers can be

262 MECHANICAL ENGINEERING FOB BEGINNERS.

answered by the help of the following formula (the formula is
transposed for the convenience of beginners) : —

W X v T» P X a:

^ = P,

= y»

W X V P X ^ TTT

— __^ = X, = W :

r ' y '

where W « weight.

^ «= its leverage.
P := pressure.
X = its leverage.

Example 1. — A weight of 20 lbs. is placed at the end of a safety-valve
lever 12 inches from the fulcrum ; the lever acts on the valve at a distance
of 1^ inches from the fulcrum ; what pressure will be exerted on the valve ?
The formula is

W X y _, 20 X 12 ,__ ,,

= P, or — T-= — = 160 lbs.

X 1*6

Example 2. —Suppose the pressure thus obtained is not sufficiently great,
and we require a pressure of 250 lbs. on the valve, how long must the lever
be, keeping the weight the same ? The formula transposed to meet this
case is

P X a: 260 X 1-5 -^ _
-.^^=y,or ^^—=IS'75.

The lever must, therefore, be 18*75 inches long.

Thermometer Scales. — The reader will find in the technical
press and elsewhere that temperatures are frequently given in
degrees Centigrade. To convert degrees Centigi-ade into degrees
Fahrenheit it is necessary to multiply by 9, divide by 5, and add
32 to the result. Thus —

100" C. 212^ F.

9 - 32

5 ) 900 ^^ convert degrees Fahren- "Too

* heit into degrees Centigrade, ^

1^^ proceed thus— P

_32 9 )900

212° F. 100°.

The reader will see from the above examples that 100° C. are
equivalent to 212° F.

Boiler Pressures. — In France pressures are spoken off in
terms of kilogrammes per square centimetre. One kilo, per
square centimetre =14-2 lbs. per square inch, or a little less

STRENGTH OF BEAMS AND USBPUIi INFORMATION.

263

than one atmosphere. It is useful to remember this, as it
enables one to make a rough mental calculation as to the pressure
in pounds per square inch, when it is referred to in the other
and less familiar way.

To Divide a Straight Iiine into a Number of Equal
Parts. — It is frequently necessary to divide a line of some
uneven length into a number of equal parts. For instance, let
us suppose that the line A B = 2|^ inches long, and we wish to

divide it into 10 equal parts. The best way to proceed is as
follows: —

Draw A C at any convenient angle and of any length. Divide
this line into 10 parts of equal length; it does not matter what
the length is so long as the divisions are all equal. Draw a line
from the 10th point to B, and lines from the other points of
division parallel to 10 B. A B will then be divided by these
lines into ten equal parts.

264

MECHANICAL ENGIKEERIKG FOR BEGINNERS.

TABLE XXII. — Decimal Equivalents op an Inch with
Areas and Circumferences.

FracUon
of an
Inch.

Decimals
of an Inch.

Area.

Circumference.

A
§
A
i

f
«
i

•0625
-125
•1876
•25
•3126
-375
•4375
-6

-5625
-625
•6875
-75
-8125
-876
-9375
1-0

•00307

-01227

•02761

•04909

•0767

•1104

•1603

•1963

•2485

-3068

•3712

•4417

-6186

-6013

-6903

-7864

-1963

•3927

-589

•7864

•9817

11781

1-3744

1-6708

1-771

1-9636

2-1598

2 3562

25625

2-7489

2-9452

3-1416

Inches.

Decimals
of a Foot.

Inches.

Decimals
of a Foot.

1
2
3
4
5
6

•0833

•1667

-25

•3333

-4167

•5

•6833
-6667
-75
-8333
•9167
10

STRENGTH OF BEAMS AND USEFUL INFORMATION.

265

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Q

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266

MECHANICAL ENGINEERINQ FOR BEGINNERS.

84 43
87-672
90-713
93-865
96 996

10018
103-28
106-42
109-66
112-70

115-84
118-98
122-13
125-27
128-41

147-26

160-40
163-54
156-68

667-26
610-26
664-84

700-98
748-69

797-97
848-83
901-26
956-25
1010-8

1067-9
1126-6
1186-9
1248-7
1812-2

1377-2
1443-7
1511-9
1581-6
1652-8

1725-7
1800-1
1876-1
1963-6

«H<

84-087
87-179
90-321
93-462
96-604

99-745
102-887
106-029
109-17
112-81

115-45
118-59
121-73
124-87
123-02

131-16
134-80
187-44
140-68
148-72

146-87
150-01
153-15
156-29

662

604-8

649-18

695-12

742-64

791-73
842-89
894-62
948-42
1003-7

1060-7
1119-2
1179-8
1240-9
1804-2

1369

1486-3

1503-3

1672-8

1643-8

1716-6
1790-7
1866-6
1943-9

•ODD

83-645
86-786
89-928
93-069
96-211

99-35
102-49
106-63
108-77
111-91

116-06
118-20
121-34
124-48
127-62

180-76
133-91
137-06
14019
143 33

146-47
149-61
152-76
156-90

556-76
599-37
643-64
689-29
736-61

786-61

836-97

888

941-6

996-78

1068-6
1111-8
1171-7
1233-1
1296-2

1360-8
1426-9
1494-7
1664-0
1634-9

1707-3
1781-4
1856-9
1934-1

r^N

83-252
86-394
89-535
92-677
95-818

98-96
102-12
105-24
108-88
111-52

114-66
117-81
120-95
124-09
127-23

130-37
133-61
136-66
139-80
142-94

146-08
149-22
152-36
155-60

551-54
593-96
637-94
683-49
730-61

779 31
829-57
881-41
934-82
989-8

1046-3
1104-4
11641
1225-4

1288-2

1362-6
1418-6
1486-1
1655-2
1625-9

1698-2
1772-0
1847-4
1924-4

e3»

82-859
86-001
89-142
92-284
95-426

98-66
101-70
104-85
107-99
111-13

114-27

117-41

120-55

123-7

126-84

129-98
133-12
186-26
139-40
142-55

145-692
148-833
151-975
155-116

546-35
588-67
632-35
677-71
724-64

77314
823-21

874-86
928-06
982-84

039-1
097-1
156-6
L217-6
L280-3

344-5
L410-3
477-6
L546-6
L6170

L689-1

L762-7
L837-9
L914-7

r*H

82-467

85-608

88-75

91-89'

95033

98-175

101-316

104-458

07-6

110-74

**-981
08-881

9X-081
80-ZXl
88-8X1

L29-59
L82-73

135-87
139-01
L4215

L45-29

L48-44
151-58
154-72

541-19

583-2

626-79

671-95

718-69

6-9Z6
88X86

8-898
989X8
66-99Z

L032-0
L089-7
149-0
L209-9
L272-4

336-5
L401-9
L469-1
637-8
L608-1

L680-0
L763-4

1828-4
1905-0

82-074
85-215
88-357
91-499
94-64

97-782
00-923
04-066
07-207
10-349

13-49
16-63
19-77
22-91
26-05

29-19
82-34
36-48
38-62
41-76

144-90
48-04
161-18
L54-33

53604
577-87
621-26
666-22
712-76

775-86
810-54
861-79
914-61
969

1024-9
1082-4
1141-5
1202-2
1264-5

1828-8
1393-7
1460-6
1629-1
1599-2

1670-9
1744-1
1819

1895-3

54S

81-681
84-823
87-964
91-106
94-248

97-389

LOO-53

L03-67

L06-81

L09-95

113 09
L16-23
119-38
122-52
L25-66

L28-80
L31-94
L35-08
L38-23
L41-37

L44'61

L47-65
L50-79
L58-93

530-93
672-56
616-75
660-52
706 86

754-79

804-25

865-3

907-92

962-11

1017-8
1075-2
1134-1
1194-5
1256-6

1320-2
1886-4
1462-2
15206
1590-4

1661-0
1734-9
1809-5
1885-7

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58ij5?5

STRENGTH OP BEAMS AND USEFUL INFORMATION.

26^7

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268

MECHANICAL ENGINEERING FOR BEGINNERS.

TABLE XXIV.— Table of Squares and Cubes.

No.

Squjire.

Cube. !

No.

Hqium».

Cube.

No.

Square.

Cube.

Mo.

Cube.

2.601

132.661 1

101

10,201

1,080,801

161

22,801

8.442,961

2,704

140,608 1

102

10,404

1,061,208

152

23,104

3,611,808

2,809

148.877

103

10,609

1,092,727

153

23,409

8,681,677

2,916

167,464

104

10,816

1,124,864

154

23,716

8,652,264

126

8,026

166.875

105

11,025

1,157,625

156

24,026

3,723,875

216

8.136

176,616

100

11,286

1,191,016

166

24,386

8,796,416

343

3,249

185,193

107

11,449

1,225,043

167

24,649

8.869,893

612

3,364

196,112

108

11,664

1,269,712

158

24,964

3.944,312

729

3,481

206,379

109

11,881

1,295,029

159

26,281

4,019,679

100

1,000

3,600

216,000

110

12,100

1,331.000

160

26,600

4,096,000

121

1.331

8,721

226,981

111

12,321

1.367,631

165

27,220

4,492,120

144

1,728

3,844

238,328

112

12,544

1.404,928

170

28,900

4,913,000

169

2,197

8,969

250,047

113

12,769

1,442,897

176

30,620

5,369,370

196

2,744

4,096

262,144

114

12,996

1,481,544-

180

82,400

6,832,000

226

3,375

4,225

274,625

116

13,226

1,620,876

185

34,220

6,381,600

266

4,096

4,366

287,496

116

13,456

1,560,896

190

36,100

6,869,000

289

4,913

4,489

300,763

117

13,689

1,601,613

196

38,020

7,414,900

824

5,832

4,624

314,432

118

18,924

1.643,032

200

40,000

8,000,000

861

6,859

4,761

828,609

119

14,161

1.685.159

210

44,100

9,261, 00

400

8,000

4,900

843,000

120

14,400

1,728,000

220

48,400

10,684,000

441

9,261

6,041

357,911

121

14,641

1,771,561

230

52,900

12,167,000

484

10,648

5,184

873,248

122

14,884

1,815,848

240

67,600

13,824,000

529

12,167

6,329

389,017

123

15,129

1,860,867

250

62,600

16,626,000

676

13,824

6,476

405,224

124

15,376

1,906,624

260

67,900

17,576,000

626

15,625

6 626

421,876

126

16,625

1,953,125

270

72,900

19,683,000

676

17,576

6,776

438,976

126

16,876

2,000,876

280

74,840

21,952,000

729

19,683

6,929

456,533

127

16,129

2,048,883

290

84,100

24,389,000

784

21,962

6,084

474,562

128

16,384

2,097,152

300

90,000

27,000,000

841

24,389

6,241

498,039

129

16,641

2,146,689

310

96,100

29,791,000

900

27,000

6,400

512,000

130

16,900

2,197,000

320

102,400

32,768,000

961

29,791

6,561

531,441

131

17,161

2,248,091

330

108,900

86,987,000

1,024

32,768

6,724

561,368

182

17,424

2,299,968

340

115,600

89,340,000

1,089

36,937

6,889

571,787

133

17,689

2,862,637

360

122,600

42,675,000

1,156

39,304

7,066

592,704

134

17,956

2,406,104

360

129,600

46,656,000

1,225

42,875

7.226

614,125

185

18,225

2,460,875

370

136,900

60,653,000

1,296

46,656

7,896

686.056

136

18,496

2,516,456

380

144,400

54,872,000

1,369

50,653

7,569

658,503

187

18,769

2,671,353

390

162,100

59,319,000

1,444

54,872

7,744

681,472

138

19,044

2,628,072

400

160,000

64,000,000

1,521

69,319

7.921

704,969

139

19,321

2,685,619

410

168,100

68,921,000

1,600

64,000

8,100

729,000

140

19,600

2,744,000

|420

176,400

74,088,000

1,681

68,921

8 281

763,571

141

19.881

2,803,221

430

184,900

79,507,000

1,764

74,088

8,464

778,688

142

20.164

2,863,288

440

193,600

85,184,000

1,849

79,607

8,649

804,357

143

20,449

2,924,210

450

202,600

91,125,000

1,936

86,184

8,886

880,584

144

20,736

2,986,980

460

211,600

97,336,000

2,026

91,126

9,026

857,875

145

21,025

3,048,620

470

220,900

103.823,000

2,116

97,336

9,216

884,786

146

21,316

3,112,136

480

280,400

110,572,000

2,209

103,823

9,409

912,673

147

21,609

3,176,623

490

240,100

117 649,000

2,304

110.692

9,604

941,192

148

21,904

8,241,792

500

260,000

126,000,000

2,401

117,649

9,801

970,299

149

22,201

3,307,949

600

860,000

216,000,000

2,600

125,000

100

10.000

1,000,000

160

22,500

3,376,000 1700

490,000

843,000,000

STRENGTH OF BEAMS AND USEFUL INFORMATION. 269

WEIGHTS AND MEASURES WITH METRIC EQUIVALENTS.

Length.

1 inch = 25*399 millimetres.
12 inches = 1 foot = 304-799 „

3 feet = 1 yard = 914-391 „

,, = -914 metre.
1,760 yards = 1 mile = 1-609 kilometres.

1 metre = 39*3704 inches.

1 decimetre = 3 9370 ,,
1 centimetre = '3937 inch.

1 millimetre = -0393 ,,

1 kilometre = 1,093 yards.

Weight.

1 ounce = 28*35 grams.

16 ounces = 1 pound = 453*59 ,,

1 ,, = -453 kilogram.

28 pounds = 1 quarter = 12*7 kilograms.

4 quarters = 1 hundredweight (112 lbs.) = 50*8 ,,

20 hundredweights = I ton (2,240 lbs.) = 1,016

1 kilogram = 2*2046 lbs.
1 gram = *0022 lb.

Abba.

1 sq. inch = 645 sq. millimetres.

144 sq. inches = 1 sq. foot = *0929 sq. metre.
9 sq. feet = 1 sq. yard = '836 ,,

4,840 sq. yards = 1 acre = *4046 Hectare.

1 sq. metre = 10*764 feet.

Volume.

1 cubic inch = 16*387 cubic centimetres
1,728 cubic inches =1 „ foot = '0283 „ metre.
27 „ feet =1 „ yard = -7645,,

cubic metre = 35 "3 148 cubic feet.

= 1*3079 „ yards.
,, centimetre = -061 ,, inch.
. gallon = *1605 „ foot.

= 277*27 ,, inches.
= 4*54 litres.

271

CONCLUSION.

A FEW words of advice from one who has been through the mill
may perhaps not be taken amiss.

In the first place, a man who has " got it all down in a book at
home " is not of very much use in a drawing office, as questions
usually require to be settled when they arise. Secondly, it is
quite inadmissible to have text-books lying around one's draw-
ing board. Engineering pocket-books containing tables of areas,
squares, &c., are, however, permitted, and as they usually contain
some blank pages at the end, it is a good plan to copy into them
any formulae or rules which the student thinks may be likely
to be useful to him. If the beginner has not got one of these
pocket-books, then he should copy into a small note-book of his
own any useful information which he may come across. Such
a note-book, to be of any real use, should be small and thin, so
that it may be carried in the pocket without inconvenience.

Amongst the information which a young draughtsman should
have available at a moment's notice, the following may be men-
tioned : — Power transmitted by belts, ropes, and shafts ; rules as
to size of steam pipes ; areas of chimneys ; head necessary to
overcome friction of water in pipes ; stress set up in the rim of
a flywheel due to centrifugal force; stresses in beams, &c. A
young draughtsman who can settle questions such as these, at all
events in connection with preliminary drawings such as are
frequently sent out, will have a much better chance of promotion
than one who is continually troubling his superior with questions
such as, "How wide must I make the pulley]" "What size
must the steam pipe heV "How large a chimney shall I
show?" &c., (fee.

The next question which arises is — How much mathematical
knowledge is it necessary for a young engineer to have in order
to succeed? The reply is — The more knowledge one has, the
better one is equipped for one's work ; but if,' in the acquirement
of such knowledge, one loses other qualifications for success,
such as health, good eyesight, strength of character, and robust
commonsense, then the acquirement of profound mathematical
knowledge may be too dearly bought.

The brain of a highly-trained mathematical man may not
unfitly be compared to a razor, while that of the ordinarily
successful engineer and man of business to an axe, which is not
easily turned aside by opposition. If an individual is compelled

272 MECHANICAL ENGINEERING FOR BEGINNERS.

by force of circumstances to go out to the backwoods, make his
own clearing, and build his own house, there can be little doubt
which of the two instruments will prove of most service. The
author has no desire to decry the razor, but, at the same time,
he would not recommend a young man with his own clearing to
make to attempt to grind and temper his comparatively blunt,
but serviceable, instrument until it had the edge of a razor,* but
at the expense of other and more valuable qualities.

The late Lord Thring stated he "found that the apparent
object of legal expression was to conceal the meaning from
ordinary readers," and although the author has great respect for
the learning of Professors of Engineering, he cannot help feeling
that the methods of many of them, while not designed to
conceal their meaning, yet are calculated to deter the ordinary
youth from studying the theoretical side of engineering at all.

Take, for instance, the following statement which appears in
a book intended for young students of engineering : — " In single

shear, the shearing resistance of the rivet is jd^f ; where f is

the resistance of the material to shearing." Would it not have
been simpler to state that in single shear the shearing resistance
of the rivet is A "x R ; where A = area, R = resistance of the

metal to shearing 1 An expression such as ^ d'^ f is all very

well for professors and others who can see at a glance that ~id^

stands for area; but if a conscientious beginner laboriously
squares the diameter of the rivet, multiplies the result by 3-141,
then divides by 4, only to find that the whole of this labour
conveys the remarkable intelligence that the strength of a rivet
equals its area multiplied by the ability of the metal to resist
shear, he will probably say that his own commonsense would
tell him that, and will struggle no further with other formulae
which might really be of use. If the beginner thinks any more
about the subject, he will wonder why in the name of common-
sense those gentlemen who are so fond of using Greek symbols
at every possible opportunity, do not choose one to represent the

area of a circle, and stick to it, instead of writing -jd-.

Again, if a student should ask a junior professor how to find
the pressure on the slide bars of an engine, he will probably be
told that the pressure can be found by multiplying" the pressure
on the piston by the sine of the angle formed by a prolongation
of the piston-rod and connecting-rod ! Now, in a drawing office
where draughtsmen's time has to be paid for, one does not find

CONCLUSION. 273

them seeking out angles by means of protractors, and then
turning up tables of sines, cosines, secants, &c. If a draughts-
man wishes to know what the greatest pressure will be upon the
slide bars, he simply multiplies the pressure exerted by the
piston by the length of crank, and divides by the length of
connecting-rod, ignoring sines and angles altogether.

As an example, showing the effect of a beginner trying to
work out simple problems by advanced geometrical methods, of
which he has only an imperfect grip, a short narrative of what
occurred in a Westminster drawing office, in which the author
worked as a junior, may not be out of place. In the office in
question, it was necessary frequently to run out the stress set up
in riveter arms. So far as the author was privileged to see, no
calculations more elaborate than those described in the preceding
chapter were ever made, either by the chief draughtsman or his
assistants. In fact, a tradition was current in the office that the
riveter arms had in the first instance been designed by a gentle-
man (not the inventor) on ultra-scientific lines, and had all
broken. However, whether this tradition is or is not true, it is
beside the story, which is this : — There came to the drawing
office, as a premium pupil, a young man who had had the benefit
of some training at a technical college. This pupil looked upon
the ordinary office methods of arriving at the stress set up in a
riveter arm with considerable scorn. He said they were quite
unscientific, and that he would show the correct way. The next
time a riveter of extra large gap, or extra heavy power, was
required, the problem was given to the pupil, as well as to one of
the older men, to work out. In the case of the pupil, the result
was as follows : — After some hours of steady work a diagram was
produced, which to the other juniors appeared to be like a small
church with its steeple. For the success of the calculations,
apparently, it was necessary to find with great accuracy the
centre of gravity of the diagram. The little church was, there-
fore, cut out in cardboard, and attempts made to find the centre of
gravity by balancing it on the point of a divider. Unfortunately,
just as the desired centre was nearly found, the office door would
be opened, a current of air would cause the diagram to fall to the
ground, and operations had to be recommenced. However, the
centre of gravity having at last been found, more steady work
was put in, a book of logarithms was freely used, several sheets
of foolscap were covered with figures, and at last the pupil said
his calculation showed that the arm would require to be (as
nearly as the author can remember) about 5,283 feet deep !
When it was pointed out that a riveter arm over a mile across
would be rather unusual, the pupil remarked, quite unabashed,

274 MECHANICAL ENGINEERINO FOR BEGINNERS.

that he supposed he had got wrong with his logarithms. The
strangest feature about this pupil was that, although he obtained
contradictory results every time he worked out the same problem,
yet he remained convinced that his was not only the right and
scientific way of working, but also the best way. Possibly it
was for one who really understood it, and had the time and
ability to work out the problem correctly, but the method in
question was quite unsuitable for every-day use in a drawing
office.

If the student does not take kindly to advanced mathematics
he need not be discouraged ; only those who have actually
worked in drawing offices of good and successful firms know
how small a part mathematics play in the daily life of the office.
The problem is rather how to get out the work quickly with a
limited staff and to avoid mistakes, than to work out elaborate
calculations.

The fundamental difference between work in the drawing
office of a technical college and work done in a real drawing
office, is that a mistake made in the former does not really
matter and may possibly pass unnoticed, while a mistake made
in the latter may involve a heavy pecuniary loss, or even give
rise to a Coroner's inquest. In the drawing office of a technical
college the omission to mark the revolving part of a certain
machine "cast steel," leaving it to be inferred that it was to
be of cast iron, might result in the loss of a few marks. In
a Midland town recently, such an omission on the part of a
draughtsman caused the loss of a couple of lives. Had the
draughtsman been accustomed to run out by simple arithmetic
the stresses set up in a flywheel rim, as described in a previous
chapter, this accident and the resulting inquest*, verdict, and
somewhat disagreeable rider, might have been avoided. Un-
fortunately many draughtsmen are deterred from making
calculations, which, although really simple, are made to appear
difficult and complicated by so many text-books.

If the young engineer should eventually have a business of his
own, his problems will be, not how to make elaborate mathema-
tical calculations, but how to get remunerative work, and, when
obtained, how to turn it out quickly and well. In many cases
another problem will be added — viz., where to find the money for
the following week's wages for his men. If a knowledge of the
Calculus would enable a young engineer to solve this problem
satisfactorily upon all occasions, there is but little doubt that
it would be studied much more widely than is at present the
case.

It must be borne in mind that, valuable as mathematical

CONCLUSION. 275

investigations are in enabling one to understand a subject
thoroughly, they usually follow, and do not precede, useful in-
ventions ; for instance, had not such comparatively "ignorant"
men as Watt and George Stephenson invented and perfected the
steam engine, many of our modern Professors, who write so
learnedly and brilliantly on the subject, would have been com-
pelled to devote their abilities to astronomy or kindred subjects.
Again, let us take the case of the problem of aerial navigation,
which, at the moment of writing these pages, has not been
solved. Books showing mathematically and theoretically how
such flight may be accomplished seem to be strangely lacking,
but as soon as some possibly ignorant and benighted inventor
constructs an air ship which can be used with safety, then,
undoubtedly, the technical press will teem with books filled
from cover to cover with mathematics showing exactly how it
is done, and the student will be told that, if he wishes to know
anything about aerial flight, he must master the contents of
these works. One cannot help feeling, however unreasonable
the feeling may be, that such books would be still more useful if
they were published before, rather than after, the problem has
been solved on other lines.

The author has often been struck with the fact that when an
engineering firm gets into a slight difficulty, say the ropes will
persist in jumping off a certain pulley, or a barometric condenser
will not give a vacuum, or possibly a boiler isolating valve
knocks itself to pieces, it is not the man with the greatest
theoretical and mathematical knowledge who gets over the
trouble, but in nearly every case it is some one having the
irreducible minimum of mathematical knowledge. The reason
for the success of the one man and the failure of the other is
probably due to the fact, not that the one man is ignorant of
mathematics, but that he can think and reason upon the facts
before him undisturbed by the noise of machinery and uncom-
fortable surroundings, while the other can only reason and think
in symbols, and when seated in a quiet room with a clean sheet
of paper before him.

In the author's opinion, there is one qualification for success
which is of considerably more importance than a knowledge of
advanced mathematics ; it is, to put it colloquially, the ability to
"keep on keeping on." It was entirely by his determination
and strength of purpose that the late Mr. Tweddell was able to
make the world see the value of his hydraulic riveter, and to
reap the financial reward due to him. The same may be said of
Mr. Parsons and his turbine, and of many others.

Competition is getting keener every day, and the struggle for

276 MEOHAKICAL ENGINEERING FOR BEGINNERS.

a competence more severe, but there is a good deal of truth in
the brave words of a modern writer — " Between aspiration and
achievement there is no great gulf fixed, only the faith of con-
centrated endeavour, only the stern years which must hold fast
the burden of a great hope, only the patience strong and meek
which is content to bow beneath the fatigue of a long and
distant purpose, only these stepping stones and no gulf im-
passable to human feet divide aspiration from achievement." It
is consoling, too, sometimes to reflect that, whether one succeeds
or whether one fails, a man cannot do more than his level best.
As Epictetus said — " It is not for the actor to choose his part,
but it is the actor's duty to play the part that is given to him to
the best of his ability, the result being left to the director of the
play." And that the crippled philosopher accepted his part cheer-
fully may be gauged from his words — " For what else can I do,
an old man and lame, than sing hymns to God? If I were a
nightingale, I would do after the manner of a nightingale ; if a
swan, after the manner of a swan. But now I am a reasoning
creature, and it behoves me to sing the praise of God. This is
my task, and this I do, nor, as long as it is granted to me, will I
ever abandon the post. And you, too, I summon to join me in
the same song."

277

INDEX.

AocuMULATOBS, Electric, 206.

,, Hydraulic, 212.

Adamson rings, 29.
Air pump, Capacity of, 169.
Alloys, 8
Alternating current, Periodicity of,

204.
Alternators, 199.
Aluminium, 8.

,, bronze, 10.

Ampere, 195, 209.
Analyses of gases, 249.
Arc lamps, 197.
Area and circumference of circles, 265.

,, of ports, 137.

Babbit metal, 10.

Babcock boiler, 39.

Balanced slide valve, 93.

Batteries, Primary and secondary,

•206.
Beams, Strength of, 255.
Bearings, Pressure on engine, 139.

Shafting, 158.
Belleville boiler, 40.
Bellis engine, 111.
Belts, Canvas, 151.
,, Leather, 149.
Bessemer steel, 7-
Blake boiler, 36.
Boiler compositions, 54.
,, Feed- water, 64.
,, flues, 44.
Boilers, 27.

,, Combustion of fuel in, 51.
,, Evaporation of, 49.
„ Galvanic action in, 37.

Boilers, Pitting in, 38.

„ Shell- type, 27, et aeq.

,, ,, dangers of, 37.

,, Steel plates for, 18.

,, Strength of, 41.

,, Testing, 55.

,, Water tube, 39, et aeq.
Bolts and nuts, 19.
Brake. 118.
Brass, 10.
Brazing, 8.

British thermal unit, 47.
Bronzes, 9.

Browett- Lindley engine, 111.
Brush- Parsons turbine, 181.

Calorific value of coals, 50.
Calorimeter, 56.
Cameron pump, 22.
Capacity of pumps, 61.
Carbon in iron, 4, 5.

,, in steel, 5, 6, 7.
Casehardening, 1.
Castings, Effect of cooling on, 11.
Cast iron, 2, 17.
„ steel, 5, 17.
Castle nuts, 20.
Centrifugal force, 131.

,, pumps, 228.

Chimneys, Area of, 52.

„ Draught in, 51.

Chromium, Effect of, on steel, 6.
Coal, Calorific value of, 50.

„ conveyers, 69.
Coefficient of friction, 139.
Combustion. Rate of, 51, 53.
Compound engine, 102.

,, ,, Calculation of

power of, 115.

278

INDEX.

Condenser, Barometric, 174.

„ Ejector, 173.

,, Evaporative, 173.

„ Jet, 165.

,, Surface, 167.
Connecting-rods, 138.
Consumption of oil and petrol, 250-
254.

„ ,, steam in engines,

103.
Conveyers for coal, 69.
Cooling towers, 174.
Copper, 8.

,, wire, Strength of, 15.
Corliss valve gear, 104.
Cornish boilers, 30, 32.
Corrosion in boilers, 37.

„ of condenser tubes, 171.
Cost of materials, 18.
Crank shafts, Material for, 5.

,, Proportions of, 139.

Cruse superheater, 67.
Curtis turbine, 185.
Cylinder walls, Thickness of, 137.
Cylinders, Iron for, 3.

„ Ratio of, 137.

Decimal equivalents, 264.
De Laval turbine, 182.
Delta metal, 9.

Diesel oil engine, 241, 243, 254.
Dished boiler ends, 43.
Dowel pins, 20.
Dowson gas plants, 246.
Draught, Inauced, 52.
„ Natural, 61.
Double-beat valves, 108.
Drop valves, 108.
Dry- back boiler, 37.
Duplex pump, 57.
Dynamos, 193.

EOCENTBIC-BODS, 138.

Economic boiler, 33.
Economisers, 63.
Edwards air pump, 172.
Efficiency of gas engines, 245.

,, ,, steam engines, 144.
Ejector condenser, 173.

Elasticity, Young's modulus of, 16.
Elastic limit, 12.
Electrical units, 196, 209.
Electric current. Production of, 193.
Elongation, Percentage of, 12.
Engines, see under Steam, Gcu,

Petrol, and Oil.
Evaporation of a boiler, Calculation
of, 49.

„ „ Cornish boilers, 33.

,, „ Lancashire boilers,

32.
Evaporative condenser, 173.

,, power of coal, 50.

Exhaust pipes, 82.
Expansion, Governing by, 134, 136.

„ of pipes, 73.

Factobs of safety, 16.

Fahrenheit degrees. Conversion into

Centigrade, 262.
Falls of Foyers turbine, 228.
Fatiffue of metals, 15.
Feed pumps, 57, et aeq.
Feed-water, 54.

, , heaters and economisers,

63.
Filters, Oil, 70.
Flanges, Table of, 80, 81.
Flash-point of oils, 254.
Flow of steam through pipes, 74.
,, ,, water through pipes, 235.
Flues, Strength of boiler, 44.
Fl3rwheel pumps, 60.
Flywheels, 129, 133.

„ Energy stored in, 133.

„ Form of arms of, 11.

,, Stress in, 131.

Forced drausht, 52.
Foundation bolts, 21.
" From and at " explained, 49.
Froude water brake, 120.
Fuels, Calorific value of, 50.
Furnace, acid and basic lining, 7.

Galloway boiler, 33.
Galvanic action in boilers, 37.
Gas engines, 239.
„ suction plant, 246.

INDEX.

279

Gas threads, 23.

Gearing, Power transmitted by, 158,
162.
„ Worm, 159.
Governing reciprocating engine by
expansion, 134, 136.
,, reciprocating engine by

throttling, 134, 136.
,, steam turbines, Curtis,

186.
,, „ intermittent,

179.
„ „ throttling,

181.
Grate area, 52.
Grover washer, 20.
Gunmetal, 8.

Hans Reynolds chain, 162.
Hardening of steel, 6.

,, ,, wrought iron, 1.

Heat lost by radiation from pipes,
78.
„ Mechanical equivalent of, 47.
Heating surface of Cornish boilers, 33.
,, ,, ,, Lancashire boilers,

32.
Helical wheels, 159. s

Helicoid nuts, 20.
Homsby oil engine, 252.
Horse-power, 96.
Howden's forced draught, 69.
Hydra-ulic accumulator, 212.
jack, 218.
jigger, 218.
lifts, 217.
,, memoranda, 237.
,, press, 211, 217.
„ ram, 235.
,, riveter, 216.
„ testing, 219.
Hyperbolic curve, 99.

Ignition for gas engines, 241.
Impact tests, 12.
Incandescent lamps, 1 97.
Indicator diagrams, 124.
Indicators, \22.
Induced draught, 52.

Injectors, 61.
Inward- flow turbine, 221.
Impulse water wheel, 225.
Iron, Cast, 2, 17.

, , Malleable cast, 4.

,, Wrought, 1.
Isolating valves, 88.

Jacketing steam engines, 114.
Jockey pulleys, 152.
Joints, Expansion, 73.

,, Lap and butt, 25.

„ Strength of, 45.
Joists, Strength of, 255.
Jonval turbine, 225.
Joule's mechanical equivalent of heat,

47.
Journals, Pressure on, 139.

KOrting gas engine, 243.

Lancashire boiler, 27.
Lap of slide valve, 91.
Lead of slide valve, 92.
Lentz valve gear, 108.
Lewis bolts, 21.
Lighting by electricity, 197.
Lock nuts, 20.
Locomotive boiler, 33.

Malleable cast iron, 4.
Manganese bronze, 9, 17.

„ EflFect of, on steel, 6.
Marine boiler, 36.
Materials, Cost of, 18.
,, Testing, 11.
Mechanical equivalent of heat, 47.

,, stokers, 67.

Meldrum's forced draught, 69.
Metallic packing, 141.
Mild steel, 2, 5, 17.
Modulus of elasticity, 16.

280

INDEX.

Mond gas, 249.
Muntz metal, 10.

Niagara Falls turbines, 228.
Nickel steel, 5, 8.
Niclausse boiler, 40.
Nuts, 19.

Ohm, 209.
Oil engines, 252.
„ filters, 70.
Open-hearth steel, 7.
Otto cycle gas engines, 239.
Outward-flow turbines, 221.

Pressure and temperature of steam,
48.

„ on bearings, 139.
Priestman oil engines, 262.
Primary battery, 206.
Producer gas, 247.
Proportions of steam engines, 137.
Pulleys, Convexity of, 161.

„ Distance apart of centres,
152, 154.

,, Fast and loose, 163.

„ Form of arms, 11.

,, Grooves for ropes, 154.

„ Jockey, 152.
Pump, Cameron, 60.

„ capacities, 61, 237.

„ Deane, 60.

,, Duplex, 57.

„ Pulsometer, 233.

,, valves, 252.

„ Weir, 58.
Pumps, Materials for, 235.

Packings for steam engines, 140.
Parsons steam turbine, 177.
Petrol, Calorific value of, 254.

,, Consumption of, 254.

„ engines, 252.
Petroleum, Calorific value of, 50.
Phosphor bronze, 9.
Phosphorus in steel, Ejffect of, 8.
Pig iron, Qualities of, 3.
Pipe flanges, 79-81.
Pipes, Steam, 71.

„ Exhaust, 82.

,, Expansion of, 73.

„ Flow of steam in, 75.

,, t) »i water in, 235.

„ Size of, 74.

„ Strength of , 78.

„ Water hammer in, 81.
Piston rings and springs, 142.

„ rods, 138.

,, speeds, 140.
Pitting of boiler plates, 38.
Ports, Area of, 137.
Power, Calculation of engine, 96, 113,
244.
,, Lobs of, in transmission, 155.
,, transmission by belts, 149.

„ gearing, 158.
„ ,« )} ropes, 15.3.

,, ,, shafts, 156.

Quality of metals, 16.

Radiation from pipes, 78.
Ramsbottom rings, 142.
Rateau turbine, 188.
Ratio of engine cylinders, 137.
Reduction of areas, 12.
Reversing gear, 92, 95.
Rivets, 24.

„ Strength of, 45.
Rolled steel joists, 258.
Rolling, effect on strength of steel,

&c., 15.
Rope driving, 153.

„ ,, Pulleys for, 154.

Rotary converter, 206.

Safety valves, 30.
Scavenging gas engine cylinders, 242.
Set screws, 19.

Shafting, Power transmission by,
156.

INDEX.

281

Siemens-Martin open-hearth steel, 7.
Siemens open-hearth steel, 7.

,, producer gas, 246.
Skew wheels, 161.
Slide bars, Pressure on, 139.
,, valve, Balanced, 93.

„ D, 91.
,, ,, Piston, 91.
Slip in pumps, 61.
Specifications as to quality, 17.
Specific gravity of oils, 254.

„ heat, 176.
Speed of belts, 150.
,, ,, gas engines, 246.
,, ,, pistons, 140.
,, ,, ropes, 153.
,, „ turbines, 181.
,, ,, Willans engines, 110.
Squares and cubes, 268.
Steady pins, 20.
Steam domes, 30.

,, engine parts. Proportions of,
137.
engines, 89.

,, Consumption, 103.
,, Efficiency of , 144.
,, Proportions of, 137.
„ Testing, 116.
Latent heat, 47.

?ipes and valves, 71, et seq*
'roperties of, 46.
-raising accessories, 57.
Saturated, 49.
Specific heat, 176.
Superheated, 49, 66, 78.
Table of, 48.
traps, 83.
turbines, 177.
Steel, 2, 5, 17.

Hardening and tempering, 6.
Hiffh-speed tool, 6.
Self -hardening, 6.
Stirling boiler, 41.
Stokers, Mechanical, 67.
Stone's bronze, 9.
Stop valves, 72, 77, 85, 86.

,, „ Obstruction of, 77.
Strength of beams, 255.
,, boilers, 41.
,, bolts and nuts, 23.
,, cylinders, 78.
,, materials, 1-15.
, , riveted joints, 45.
Stress and strain, 12.
Studs, 19.

Suction gas plant, 246.
Superheated steam, 49, 78.
Superheaters, 66.
Surface condensers, 167*

Teeth of wheels, 159, 163.
Temperature, effect on metals and

alloys, 16.
Testing boilers, 55.
,, elongation, 12.
,, impact, 12.
,, machines, 11-13.
, , materials, 11; see also Tables

I. and II.
,, steam engines, 116.
Thermal efficiency, 145, 245.
, , storage system, 65.
Thornycroft boiler, 41.
Threads, Whitworth and gas, 22, 23.
Throttling, Governing by, 134, 136.
Transmission of heat, 54.

,, ,, power, 149.

Tungsten, Effect of, on steel, 6.
Turbines, Economy of, 190.
„ Steam, 177.
„ Water, 221.

United States packing, 141.

Vacuum augmenter, 171.
Valve, Balanced, 93.

„ Corliss, 104.

„ D-form, 89.

„ Drop, 108.

„ Hopkinson-Ferranti, 87.

„ Isolating, 88.

,, Lentz, 108.

,, Piston, 91.

„ rods, 1.^8.

„ Stop and gate, 72, 85, 86.
Valves and pipes, 71.
Van der Kerchove engine, 108.
Vertical boilers, 37.
Vibration of engines, 111.
Volt, 195, 209.

282

INDEX.

Water brake, Froude, 120.

hammer, 81.

-tube boilers, 39.

turbines, 221.

Weight of, 237.

wheels, 220.
Weir pump, 58.
Welding heat, 1.
Westinghouse turbine, 188.
Wheels, Raw-hide, 161.
„ Toothed, 159.
„ Worm, 159.
White metal, 10.
Whitworth compressed steel, 5.

,, threads, 22.

Willans engine, 108.

Willans-Parsons turbine, 180.
Wohler's law, 16.
Worm gear, 159.
Worthington pump, 57.
Wrought iron, 1.

Yakrow boiler, 41.

Young's modulus of elasticity, 16.

Zeunbr diagram, 145.
Zoelly turbine, 189.

OF THE

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